45acp is cheaper to shoot,
Where are you buying your ammo?!
When I still had my SKS I'd buy ammo for no more than $2/box of twenty for plinking. The good hunting ammo in 7.62x39 Soviet costs less than $15/box. But the last time I bought a box of Hydra-shoks for my .45 they cost me $16 for a box of twenty.
And for deer I'd say that the 7.62x39 is a better choick than .45ACP any day. While the .45ACP would certainly kill a deer if you hit it right, it's not going to have the range or hitting power of a good .311cal bullet at 2000+fps.
Now if we were to talk about the "fun factor" of the two, I'd definately have to side with the Thompson. I think it'd just be a joy to shoot, especially if you could get your hands on a pre-ban drum magazine. It'd make a good home defense gun too.
Mike