I have no experience with CodeWarrior, but they seem to target embedded platforms.
I would add PathScale to the list, they have x86 compilers, and used to be our favourite in the past with AMD systems. But lately it is all Intel.
Also the IBM compilers should be good, but not available for x86. So assuming you have access to a POWER or PowerPC machine, you can only compare it to GCC on the same machine.