Hi, I'm currently a UCD senior, if you have any specific questions feel free to ask me!
I like UCD.. and not all parts smell like cow
... although the area near Tercero (one of the freshman dorms can smell when the wind blows a certain way during spring)..
My sister went to UCI and so I've been to their campus as well, I think UCI's campus is really nice, really new looking while UCD seems more.. worn in I guess. I like UCD though because its so laid back and everyone here is so nice! (at least I think so..).. the student environment is very non-competitive and people are just really nice overall.
Oh and I didn't want to give the impression that UCD is just a barn or anything, we have a lot of new science facilities (new labs for all general chem students, new labs for ALL lower division biology courses, new labs for Microbiology, new labs for all physiology courses, etc). New Science lecture hall. We are also making the transition to DI sports so if thats your thing UCD might be a good fit. We're actually going to be opening our new stadium next year (should be exciting, too bad I'm not going to be here.. I'll be in pharm school
) Our library is huge and its pretty nice. We also have the new Recreation center (the ARC) like Eddie mentioned and we have 2 new freshman dorms and new dining centers for freshman. And then there is the Mondavi center.. ok ok I'll stop now haha..
Both schools are great for Bio so that probably shouldn't even be much of a factor in terms of which school you go to (unless you are looking at a specific emphasis maybe..). Its basically which school's overall atmosphere fits what you are looking for.. like other people have posted.. UCD is a laid back college town where everyone and their roomates bike to class, while I think UCI is more of a commuter school and people go to surrounding LA hotspots to have fun. If you LOVE and NEED the city life Davis probably isn't for you, but if you want a laidback, chill college experience go to Davis!
You'll get a great education either way.
And in terms of rankings Davis and UCI are pretty neck and neck (along with Santa Barbara). I think Davis was ranked really low one year (not sure if it is the current rankings), because someone accidently sent USNEWS wrong data saying 65% of professors have PhD's while the real number is like 97% or something, so it really hurt the rankings.