If you are looking to buy your first and only black powder gun (muzzle-loader) I'd recommend going with a .50 caliber. I bought a .45 this year but I already owned 2 .50's
Besides, the pyrodex pellets are cheaper (per grain) when purchased in the .50 caliber size and the .50 bullets are currently a little easier to find most places. The extra range offered by the .45 is pretty much offset by the extra punch of the heavier and broader front of the .50
If it's going to be your first and only I'd go with the .50