CarmenCarminis

First, congrats on getting accepted into both of those! Having to chose between schools is a good problem to have, and those are both top-notch programs. 
If you like a similar number of faculty at both, I'd probably go with the one that you feel more comfortable at. If you felt like you clicked better with the students and faculty at UCSF, then I'd trust that instinct. These are going to be the people you spend the next several years with, after all, so I think it's important you find the right fit. 
I would try not to worry about not being good enough (which I realize is easier said than done). That being said, there's nothing wrong with choosing the "safer" option if that's the better fit. (Grad school is going to be stressful enough either way.)
But both are great choices, and I don't think there's a "wrong" answer here.