I honestly think they're about equal in terms of ability, it seems McCown can throw a nice ball as well. I'd go with Cutler because I think there's a good chance he comes out rusty, and injury prone. He came back against the Lions, and he ended up getting hurt late in the game if I recall correctly.
Though, it shouldn't matter who's the other QB if Romo and our offense come out firing. I don't think neither of those QBs can keep up with our offense if we're firing on all cylinders, though we shall see.