I say black, because that's the color this fall and I think you'll look really current with a black one (though the fawn is also lovely, and would probably also work with black). It's a tough decision! The black might also show less wear over time... my gosh, I'm such a black bag whore! I'm always so drawn to black.
I say get whatever bag you feel more drawn to and don't think too hard about it... go with your gut (it's so ironic that I'm saying this! I always over think these choices.. I can tell you, though, that I always go back to the bag I had the strongest feelings towards in the first place, even if I told myself again and again that the other color was more practical/useful/trendy).