whistler is quite an experience, just because of it's size. You can ski one mountain for a week then go to blackcomb for another week of completely new trails and slopes.
Whistler is renowned for that size and because of some nice backcountry stuff that looks good in magazines. The actual skiing all depends on snow cover, tempurature, and season. Which is pretty much the same in louise or sunshine. The largest difference between these mountains is that because sunshine and louise are furthur EAST, a lot of precipitation has already fallen by the time it reaches these places and snow cover is usually a few meters less than in whistler. Early season skiing in louise or any western rocky resort can be a risky proposition for you and your skis is you go off the groomed trails.
Once there is suitable snow cover at these two fabulous resorts, the skiing is incredible. If you are a beginner or intermediate rider, both mountains offer tons of terrain to go play on and have fun for the weekend. If you are an advanced rider, Lake Louise is generally the better choice, even with Sunshine village opening goat's eye mountain. There just is more advanced terrain at louise, plain and simple.
Louise is my personal stomping ground, there are a few great backcountry areas you can ski out to a lift and a couple nice cliff/cornice drops that are a lot of fun. Also, I haven't seen sunshine's latest attempts at a terrain park, but I know Louise has a great setup with a full superpipe.
if it's worth it for me to drive out from Edmonton at 3AM to go skiing there it is definetly worth the trip to go ski these resorts from Calgary, a much shorter drive.