preface: I'm not a UCI or UCSD student, but one of my friends faced the same decision...
UCSD is quite a bit more hard core, requiring all students to do research. And I've heard that they're rather unkind in their dean's letter when you apply for residency. But hey, you're in San Diego...
UCI is more laid back, and their residents seem pretty happy, so it makes for a much nicer working environment once you're on your rotations.