First off, congratulations! I'm sure you know that they are both great schools, and in many ways they will offer similar opportunities to you. They also both seem to have very high student satisfaction. How much will cost be a factor (ie, are you a CA resident)? If you are a CA resident, or are planning to try and become one, you will save a LOT of money going to UCSF. Then, there's quality of life. San Francisco can be a very fun, exciting city. Cost of living is probably higher than New Haven, but rents have been going down this year in SF. And of course, there are the differences in curriculum. How important is the "Yale system" to you?
Since the schools are very similar reputation-wise, personally it would boil down to a quality of life issue for me.
Basically the question boils down to what school is a better fit for you. Think about what you ideally wanted in a med school, and see which school most closely matches up with that image.