<< The main difference is basically the emphasis each school has. Most comp sci people I know are at Warren. >>
I hate the way this isn't explained...
UCSD's 5 college system (soon to be 6 college system) does vary by "emphasis," but all that means is you have to take different GE's. You can take any class or be any major from any college. Also, each college has its own "writing" sequence of classes that varies from college to college.
To graduate from UCSD, you need the following:
Your college GE requirements + cultural diversity + requirements for your major(s) + at least 72 upper division units and 180 total units.
As far as each college's GE's go...
MUIR: the least requirements and, in my opinion, best atmosphere to live in. It's writing program, Muir Writing, is lame, but only 2 quarters long and pretty easy.
REVELLE: lots of requirements and a required course of study outside your major. Their writing program is called HUM (Humanities). It's 5 quarters long and a lot of work.
WARREN: a good amount of requirements. They require 2 areas of study outside your major. Their writing program, Warren Writing, is not too bad and only 2 quarters long.
MARSHALL: not too many requirements. Their writing program, DOC (Dimensions of Culture) is 3 quarters long. Marshall college has a lot of programming focused on different cultures.
ROOSEVELT: lots of requrements, including MMW (Making of the Modern World), a SIX quarter writing sequence. It's a lot of writing and much complained about. The rest of the requirements are pretty standard, but MMW is lots of work.
Anyway, UCSD is a great place and while I think Muir is the best by far (I'm an obviously biased Muir graduate), the school as a whole is wonderful.
Hope that helps.
-geoff