Mule deer's taste depends a lot on what its been eating and if its a buck in rut. Since mule deer eat a lot of sagebrush, I have always found the taste to be stronger (elk is milder by comparison). Whitetail is usually milder as well. However if the mule deer is young and has been feeding on grain fields, the taste will be milder.

One of the best ways I have found to take out the wild gamey flavor of any game, is to soak it in buttermilk overnight, then rinse & dry the meat and prepare according to your recipe. Some folks like to use vinegar based marinades, but I personally feel that vinegar changes the flavor of the meat too dramatically. The buttermilk simply removes some of the gaminess and tenderizes it at the same time. Recipes with tomato in them work well too, actually any recipe with something acid in it will both tenderize and curb the gamey flavor.

I can't compare to whitetail because I have not had it. However if you shoot an older muley it can be pretty "sagey" or whatever they have been feeding on.

I haven't eaten a lot of mule deer and never had a problem with it, but if you shoot an older buck it might be kind of strong in taste.

Mule deer is pretty good eating. My family love it. I got a buck this year I will post a link or picture however it comes out on here. But I took this buck just as the rut kicked in and he was the most tender animal I have ate before. My friend eddie's dad said he would rather have the buck the I got than a doe or fawn due to since he was just coming into the rut he was tender. He said all they do before the rut is eat and sleep and havent got all worn out from chasing them doe's. So my buck was extremely tender. He had a 28" neck when skinned out. So he was coming into it full force.
http://www.hunting-pictures.com/members/supersider34/Big%20buck%202002.jpg