I've always preferred clang for its better lints, and friendlier error messages (sans some decrepit parts around templates that are equally horrendous everywhere)...
And theoretically clang has better ASM output in some cases I say theoretically, because it's been shown that GCC's "worse" ASM performs better; I'm not really an architecture aficionad, so I can't comment as to why that is.
Also, it's been a few years now since I did C/C++. So, maybe these are no longer the case.
Anyway, I've kinda pointed out what I like about clang over gcc, but I'd be curious what you prefer in gcc.