Palo Alto is in a nice area, but so bland it's almost tasteless. Philli would be a fun experience if you've never experienced seasons or the east coast mentality and LA is a big city in the sun. Where would you want to live.
I'd also be a little wary of Stanford's size and location. It's a small population size and full of yuppies, not so conducive if you see yourself going into ED.
This is the relatively unfounded reputation that Stanford gets because it is located in a very beautiful college town with pretty much continuous perfect weather and very low crime. Just because the town is like that, doesn't mean that reflects your clinical exposures.
The Stanford hospital is a big tertiary care hospital (613 beds) that gets lots of complex cases (e.g. a big center for heart transplants, bone marrow transplants, the most complex neurovascular neurosurgeries like moyamoya disease, etc.), and it is undergoing a massive expansion and the whole hospital complex is under construction. It also has a large very modern children's hospital (Lucile Packard Children's Hospital). Lots of adult and pediatric trauma are directly airlifted there via Life Flight from all over Northern California. It's important to remember that emergency medicine isn't just penetrating wounds from guns or knives, but accidents from rock climbing or skiing, burns in industrial accidents, construction injuries, frostbite and hypothermia, etc, all of which you can see in abundance because of the variety environments around Stanford, where surfing is a half hour drive west, and skiing is a couple hours drive northeast.
However, med students do their clerkships split between the Stanford hospitals, the Palo Alto VA, and Santa Clara Valley Medical Center (SCVMC). The Palo Alto VA has ~900 beds and serves a huge chunk of Northern California veterans is a major site for treatment of soldiers wounded in the Middle East (e.g. brain & spinal cord injuries, PTSD). SCVMC has ~575 beds and is the primary hospital for San Jose (a city even larger than San Francisco), and you can see a wide range of emergency cases in that big city environment. This compares very favorably with the UPenn's 776 bed hospital. It's a great hospital as well that also receives a lot of it's emergency cases coming in via helicopter from a wide area.
Lots of students also do rotation clerkships abroad, and there are many partnerships and programs for those opportunities around the world.
One of the great things about Stanford is that the research opportunities are not just what you find going on in professors' labs, but there is tons of infrastructure, mentoring, and financial support for any projects you dream up. If you are interested in technology or tinkering, there is a whole phalanx of people around to help turn your ideas into something that gets out into the market and into clinical practice. At the same time, there is a huge amount of money and interest in all kinds of international and domestic development efforts, what they are calling "social entrepreneurship".
If you can come up with an interesting research or service project, you have to really go out of your way through apathy or laziness to avoid getting your ideas funded. You also get to interact with people outside of medicine who are transforming the world in dramatic ways, whether through technology, at the business school, or in other disciplines.
