RE: More challenging Whitetail or Muley
To just put your tag on one, anyone, a whitetail is easier just because there are so many more and they typically live in more confined spaces than a muley.
That said, after a couple of years in Colorado, I think i have stalked within bow range of more mule deer than I ever have whietails. They seem to either not be as wary, or if they do spook, they run 30 yards and look back at you waiting to get shot. A whitetail just wont do that. No experience dealing with trophy mule deer bucks though.