It really depends on what you want to do..hiking, skiing/winter summer etc. I have been to Whistler many times and going back there in August this year to play golf at Nicklaus North, however, I lived in Banff for a few months going back 30 years when I played football there for a season. Banff is really pretty, and every corner you turn you feel like its a photo opportunity. Whistler is pretty also, but in a different way. If I had to select a destination I do favour Banff only slightly over Whistler.
At Whistler in summer the village is taken over by mountain bikes, and even when you take the Whistler Express to the top of the hill, it is nice, but not as nice as Banff. The drive into Banff is particularly nice also. There are some great restaurants in both places, but Banff has had a lot more work done to it; feels a little bit more modern and is like a little town whereas Whistler has that village feel. I was living in Banff when a restaurant called Melissa's was the first licensed establishment in Albert to allow patrons to stand when they were drinking. Now that's a long time ago, and you had further to fall at the end of the night.