I just realized how good the Thompson is in World at War. The only thing I don't really like is it's low clip, but it's got a high fire rate, high damage, and it's very accurate. When it's silenced, it can still take people out in a short amount of bullets.
That's the only problem in World at War. All of the SMGs are awesome with any attachment, but the MP40 is a big spray and way kind of weapon.
The Thompson is more ammo efficient, but it doesn't really matter considering the game doesn't keep track of your accuracy anymore.