Congrats on getting offers from both schools! I agree with the others that this is a great problem to have. I'm a 4th year WashU student and I interviewed at both places for residency and ranked WashU higher (and matched here!). Here are some of my thoughts on this:
1) Reputation of schools. I agree that they are roughly equivalent but being a WashU student, I'm a little biased =P. Both are research powerhouses. People inside medicine recognize both UCSF and WashU as awesome schools. People on SDN seem to look a bit more favorably on UCSF but I wonder how much that has to do with the actual strength of the programs and how much is colored by people's views on the cities. From what I know of my own field and my friends talking about their fields, there are some residency programs that are better at WashU than UCSF and vice versa. You won't really get a sense until you talk to people in your specialty of interest. People outside of medicine might recognize WashU but won't know anything about UCSF since they don't have an undergrad, but this doesn't matter much. WashU matches people very strongly to both coasts and the midwest and we are enthusiastic about sharing our match list. I honestly haven't seen UCSF's match list posted for the last few years but I'm sure its great too.
2) Location. Ok, I will admit it. San Francisco is awesome and I loved my visits to that city. If you are from California, I could easily see how you could prefer the natural beauty and weather of the bay area. If you have a spouse or significant other who will be moving with you, it will be easier for him/her to find a new job in SF than in St. Louis too because it is a bigger metro area with more tech jobs. That's just the way it is. That being said, St. Louis and especially the area around WashU is probably the most under-rated location in the entire country. We are situated right next to the largest city park in the country and the world class art museum and zoo that are located in the park are entirely free. The Central West End is a great walkable neighborhood with lots of young people and several other neighborhoods next to the medical campus are rapidly improving with lots of new construction. You could get by without a car on a daily basis but having at least a friend with a car is still necessary for getting groceries until they finish building the Whole Foods in the neighborhood. St. Louis does have some issues its working through but in my opinion, it has a bigger image problem. North St. Louis has some urban blight and crime but as a student, you would never have to go there. Total crime statistics are blown out of proportion by the fact that St. Louis City only makes up about 1/3 of the population of the metro area and contains most of the roughest areas. The actual crime statistics for the St. Louis metro area show that it has fewer crimes per capita than the Vegas, Atlanta, Philly, Los Angeles, San Francisco, or Washington DC metros. A recent Gallup survey showed that 74% of St. Louisans report that they feel safe walking at night in their areas, thats a higher percentage than residents of Philly, Nashville, San Francisco, and Charlotte. I can think of tons of major med schools that are located in worse parts of their cities than WashU. The political boundaries also distort how big St. Louis actually is. The Metro area has a population approaching 3 million and has never declined in population. It is the 21st largest metro area and is comparable in size to places like San Diego, Portland, or Denver. The city of St. Louis only takes up 70 square miles and if you add St. Louis county to make it comparable to the surface area of other cities, it would be the 8th largest city in the US, between San Antonio and San Diego. The light rail trip from the airport to the CWE goes through some of the rougher parts of northside and I think that reaffirms many people's preconceptions that St. Louis is some sort of crime ridden, dead town. I don't blame the visitors though, I blame the city for not putting our best face forward. Years of bad luck and poorly chosen political boundaries have given us an undeserved reputation. The winters are milder than up in Chicago or the Northeast, although our summers can get hot. Unlike the west coast, we have all four seasons which you can see as a positive or negative. We have the Ozarks nearby but they are not as great hiking spots as the mountains out west. St. Louis is a city with lots of history and is older than the US itself. The city itself has some of the coolest old brick architecture you'll find anywhere, and tons of units are getting rehabbed and brought back online every day. The downtown district has greatly improved too and its residential population has increased 200% in the last 10 years. If you're looking for a place with some grit and want to be part of a revitalizing city, this is the place for you.
3) Cost of living. Let me assure you, unless you are independently wealthy as a med student, you can't afford to live as well in SF as you can in St. Louis. Heck, I did some calculations and even on a resident's pay, I could barely afford to live in SF. The differential between my mortgage on a townhouse 10 min walk from where I'll be working at WashU, and the rent for some equivalent sized place in a... suboptimal location in SF would allow me to fly to SF or anywhere else in the country at least three times a month for vacation if I wanted to. For some people, the difference in cost of living is worth it. For others, its not. For me personally, it clicked in my mind one day that even as a resident, I could live the life that I'd always dreamed of: I'd own my dream home, I'd walk 10 min to work, I'd save the money to travel anywhere I want, and if the only catch is I have to live in some place called St. Louis, thats a sweet deal.
4) Focus. WashU is the place to be if you want to become a specialist or researcher. I'm not saying you won't be successful if you want to do family medicine coming from WashU. If you're interested in primary care, you'll get all the support and resources in the world, and one of the folks in my class matched into family medicine at UCSF this year. However, the fact that we don't have a department of family medicine, and we don't require you to rotate in family medicine should say something. UCSF, to my knowledge, makes primary care and FM a bigger focus of their education but they have some great research going on there too.
5) Facilities. UCSF's Parnassus medical center has some of the most breathtaking views of any place in the world but the buildings themselves are old and in terrible need of either major renovations or demolition/reconstruction. SF general hospital is the same way. UCSF's facilities are spread out across the city with some other sites such as Mt. Zion and Mission Bay. As mentioned, in the UCSF thread, you will have rotations spread out all over the place. All of WashU's main clinical buildings are located in the Central West End and with a few minor exceptions, are well kept up and modern. One of the coolest things that happened on my WashU interview day was a faculty member showed me where he worked for clinic and where he had his lab and they were a few minutes walk away from each other. Awesome. The Barnes/Washu campus is getting more than a billion dollars worth of new construction in the next 10 years so we are already looking to the future. I've seen many major medical centers in the US (did I mention I can afford to travel?) and I can say that the only ones that impress me more from a pure facilities standpoint are some of the Harvard hospitals, Texas Medical Center, and maybe Cleveland Clinic. Although Barnes Jewish owns several other hospitals in the St. Louis area, you could do all your rotations at the main campus if you wanted to. You can't discount the awesome study spaces and lecture halls we have here either. The biggest facilities complaint you'll get from WashU students is that our EMR is kind of fragmented and our medical center is so big that it can be hard to find your way around. We could also probably use a multi-floor fitness facility like Mayo has to make our staff members more fit.
6) Random Intangibles. Many people don't realize this but there are some schools that have awesome match lists because a huge proportion of students spend extra years on research. Not sure if this is true at UCSF. WashU has a strong match list and outside of the MD/PhDs, only a few choose to do extra years. Having H/HP/P/F for 2nd year isn't ideal and it does stress some people out but your grades during 2nd year don't really matter in the long run. I got about half HP and half H with some scattered Ps in 2nd year, which I'd say is below average for preclinical grades and I still got great step 1 scores and my choice in a competitive residency. WashU is really rich (see construction noted above) but also is liberal in funding opportunities for you. Most people who apply for summer research get funded with a stipend. My friends and I wanted to go to a conference in Toronto, we got funded. I wanted to go attend a conference with friends in Chicago, money appeared. I wanted to go to Palm Beach to present, money. Had a presentation in Boston, money. I got to the point when I was embarassed to ask for more money because I'd always get it. Granted, not all of these were through the administration but often, I'd just asked my research mentor and he'd gave me funds to travel. Also, many schools have required rotations in 4th year (also not sure about UCSF) but at WashU, you can literately do whatever rotations you want in 4th year and you get 2 months of vacation that you can take whenever you want. That makes life so much better so you can get the rest you need before intern year. Its their gift to us for stressing us out and giving us grades to make us learn in 2nd year.
Please PM me if you have any questions about WashU or St. Louis. I'm not linked to the administration or recruiting in any way. Just a 4th year very happy with his education here and with life in general right now.