Hey guys,
I'm new to this forum. I thought I'd share my biased input (I'm pretty set on going to UCLA next year). I'll break down my UCLA-bent into the categories of med school, research, and student life.
Med school: UCLA has the more relaxed learning environment. I don't want to be ultra-stressed out in med school.
Research: UCSD has Salk, Scripps, etc., a lot of opportunities, but so does UCLA (and Caltech, if you are part of that program). Like ppl have said countless of times (I particularly like the micro-environment example), a good mentor is vital, and you can find that both at UCLA and UCSD.
Student life: UCLA and UCSD are going to be expensive. Keep in mind, however, more young people and more activities will be present in the West Hollywood area. LaJolla is beautiful, but only old, rich ppl can afford to live there. In either case, you probably will need a car in SD or LA.
hope that helps!