One other thing to consider is the quarter versus the semester system. Personally, I really liked UCLA's quarter system. You get to take more individual courses this way...leaves you lots of room for exploration. If you don't like a course or a prof, you're only stuck in it for 10 weeks.
It goes really fast though. You have to stay up on your studying because mid terms and finals come up super quick. On the good side, you only have 10 weeks worth of stuff to learn for the final instead of a whole semester's worth.
Does Cal have better or more unique research opportunities than UCLA? I have no idea...even if they are "slightly" better than UCLA's, it's not exactly like UCLA's are non-existent. It would really be splitting hairs here.
Is Cal having a "Bear Day"? If so, you should go. Have you been up to Cal before? Thoroughly checked out the area, the campus, etc? If not, you need to do so.
Honestly, you can't go wrong...both are great choices. I have several friends from each..most of whom had to make the exact same choice you are making. All were happy with their choice. Go with your gut. Good luck.