GCC has caught up a lot with clang in the last few years, from the quality of error messages, to standard compliance, compilation times, tooling, sanitizers, etc.
IME GCC tends to produce faster code in practice, even if clang is often able to out-smart GCC in many areas (e.g., IIRC heap elision was first introduced in clang). GCC has a huge amount of inertia behind it, it's still the only compiler officially supported for compiling the Linux kernel, and, as long as the GCC developers don't stop improving it, it will be difficult for clang to take its role as "default" compiler, at least in Linux environments.
I use them both daily and I am really happy that we have the possibility of freely choosing between two such amazing projects. It would be a step backwards if one of them ended up fading away.