Hmmm... I'm not sure what's up with people encouraging you not to go to Stanford. It's a great place. I can't tell you about the Northwestern program, but I can tell you about Stanford. I graduated in '99, took a year off, and now I'm headed to med school in the fall.
Stanford is going to give you tons of opportunities to do pretty much anything you want. Research, clubs, volunteer opportunities....you name it and it's generally available (and you can usually get funding for most projects). One of my biggest problems was trying to limit all of the things I was involved in. You can choose E.C.'s you you will really enjoy and still look good to med school admissions committees.
You'll love the weather out there. I moved to Palo Alto from Colorado, so I ended up wearing shorts year round. It's a short drive to SF and all the social stuff up there, or head down the coast to Monterey for a weekend. While snow is great, I enjoyed my 4 year vacation from snow boots and digging out my car.
I have to say one of the best things about Stanford is that about 90% of the students live on campus (a combination of dorms, houses, and apartments), which gives the students a closer feel. When everyone is spread out throughout a town, you don't get the same group dynamic you get from living on campus. Still, you sometimes tend to forget that there's an outside world, which is why we sometimes call Stanford the "bubble".
As far as classes go, most premeds either major in Biology or Human Biology. The bio classes are pretty straight forward - science, science, and more science (this is where most of the hard cord premeds go). The Hum Bio track is more flexible. You still take all of the core bio classes, but about half of the major is up to you to design. I chose to focus on the history and ethics of medicine, but other's did things like molecular biology, genetics, social impacts on whatever, etc. The core classes are fairly hard, but they will definitely prepare you for the MCAT.
There is an excellent premed program office to help you from day one. If you talk with them from day one, you'll get about as much help as you want (advice on classes, letters of rec, practice interviews, etc.)
The one negative would say about Stanford's premed environment is that it has a lot of gunners. There were quite a few premeds that were a little over the top with grades, and always needing to be the best, but I suppose you'd get that about anywhere you go.
All in all, I loved my time at Stanford, and I was given every opportunity to learn and prepare for med school. Then again, just because it worked for me, doesn't mean it'll work for you. Try the admit weekend if you can. You'll get a feel for the students and the campus as a whole, which is invaluable when making your decision. I'm sure NW is great too, and I'm sure you'll be happy either way you go.