Both the .45 & the .50 cal are good calibers in a muzzleloader for deer sized game.
As two of my fellow peers on this site have already stated "speed kills" & "shock kills". While both of these elements are important in a humane kill, I believe that shot placement out weighs the other two mentioned factors.
No matter what caliber you chose, being a responsible hunter involves studying the anatomy of the game species you are hunting, learning to judge distance, firearm safety, and taking your muzzleloader to the range and practice, practice, practices. Good Luck.