1. License. If you are worried about GPL (especially v3), then you are better off with clang.

2. GCC is generally considered to produce better binaries.

3. Smart projects will support both and test with both. Before, even though projects tried to be portable, GCC was the only compiler most developers had available. Now they have two (although clang does try to support almost everything GCC does, so it doesn't say a huge amount about portability).

4. If you want to quickly see whether an optimization was applied (like inlining or loop unrolling), looking at LLVM is more approachable, so clang is nice for that.