From what I can tell, Clang+LLVM is where all the innovation is happening. Clang in particular has seen many new security features for a modern compiler like the implementation of SafeStack[1] and Control Flow Integrity[2]. Even just on the functional side of things Clang has thinLTO[3], which makes building link-time optimized binaries easier on lower-end systems. None of these features are currently planned for GCC.

[1] https://clang.llvm.org/docs/SafeStack.html

[2] https://clang.llvm.org/docs/ControlFlowIntegrity.html

[3] https://clang.llvm.org/docs/ThinLTO.html