More research style projects are indeed based on LLVM, but GCC is extremely serious about security too, (and contains more Linux-specific optimizations & code too).

For the end user, it comes down to - do you care about copyleft and what FSF/Stallman are advocating for, or do you just want a free (as in beer) code, (LLVM).

I'd argue as a user, the GPL cares more about your freedom, so unless you have a specific reason not to use GCC, go with that over LLVM, (yes, am aware of the exception re binaries compiled with GCC not having to themselves be GPL).