Llvm is better than gcc at modularity and extensibility (or at least it was when llvm was released, I haven't followed gcc evolutions in a while). People who work on new languages typically use llvm because it's designed to make such things simple.

Now, in terms of end results, llvm and gcc each have their qualities. When llvm was released, gcc typically produced faster binaries but llvm optimizations were easier to understand. Since then, both have evolved and I haven't tried to catch up.

Bottom line: having two back-ends for rust or any other language is good. Among other things, it's a good way to check for bugs within an implementation of the compiler, it can be used to increase the chances of finding subtle bugs in safety-critical code, etc.