I found that for CQB, a pistol and a paintball mask works just fine. Realistically, I wouldn't bother with an auto pistol, I've seen a lot of them break and they seem to have a lot of problems. One of the PDWs or SMGs mentioned above are good if you need full auto, but a good pistol also works, if cost is something you're worried about. Just make sure you get a couple of extra mags.
That way, you can use the paintball mask, it'll provide superior protection, and you don't need to worry about cheekweld. This solution isn't as effective outdoors, though, so keep that in mind.
- Fides et Amicitia