If all you are going to hunt is deer, I would get the .45cal. Looks like it has a slight velocity adgantage, which equals less arc in the trajectory and less wind drift. If you are thinking that you may go after bigger game, then you need the bigger bullet that the .50 can throw.

I like the loose powder, you can adjust the charge weight to anything you want. I also like that it will fill the breech plug when you load it, so there isn't a gap between the percussion cap and the powder.

That said, if I was going to get a new gun I would probably get the Optima in .45cal and put a nice 3-9x scope on it. A friend just got one from Wal-mart for $200 (OK $199.95), then he put a Simmons 3-9x scope on it, and the whole works was just under $300. Then I would get together some different bullet/sabots and test them to see what shoots best.