I've only been to Louise and Sunshine. They're both great in their own ways.
Louise has a lot more steeper terrain, a bit longer runs, some great scenery and I think it has a better park (don't quote me on this 'cause I haven't yet skied through the whole thing yet). On the downside, the frontside of Louise is usually icy, so stick to the back bowls for the best snow. Also Louise typically gets less snow than Sunshine and it ices up quicker as well.
Sunshine's biggest draw is that it typically has much better snow than Louise, as it is at a higher elevation. There's still a large variety of runs and if have a buddy and avalanche gear you can run the dive which looks pretty neat. On the downsides the runs are usually not as long unless you know how to string them together and there's lots of flat spots which means you might be walking a bit if you come to a stop.
Theres also Norquay but I heard it was pretty badly off this year. I've never skied it so I cant tell you really much about it except it has night skiing (some green runs and a park).
Was at Sunshine today and snow was ok (a bit scratchy but still workable. At noon it snowed about 3 to 4 cm which was enough to cover up a lot of the harder areas - this happens often at Sunshine).
Skiing Louise tomorrow so I can let you know more later.
Overall: expect snow to be better at Sunshine, but better terrain at Louise.
If you have a chance... ski them both!