If you still have the time and can afford the ammo/powder & bullets, I'd borrow both, then go to a range and see which one works best for you. Probably the easiest would be to borrow the shotgun and buy some slugs. Either way, make sure you check it out at a range to see where it's shooting to avoid being disappointed when that deer walks out in front of you. Borrowing a gun is always a risk, since you don't know where it will hit at any given distance. Either of these choices will take down a whitetail.

Good Luck