I stayed in Canmore on a trip last year, and if you have a rental car it won't be a problem. Figure on adding about 15 minutes of travel time onto whatever the resorts state for travel time from Banff. The drive is easy, along the well-travelled and well-maintained Trans-Canada highway. Canmore is outside the park gates, but once you have your vehicle pass you don't need to stop... you can cruise right through.
I would describe the big 3 as follows:
Mt. Norquay/MR - About 5 minutes from Banff, and not nearly as big as the other two resorts. They get less snow, and have less vert than the other resorts. HOWEVER, they offer some steeeeeep cruisers that are great for a few quick runs on day you might partially spend doing something else. They sell hourly lift tickets so you can pay for exactly how long you want to ski.
Sunshine Village - About 20 minutes from Banff, with the best snow in the region. Sunshine will have the best snow during the Christmas season and can usually open their terrain up pretty quickly in a good year. Although they list 3000+ vert, it's more along the lines of about 1500 or 2000. (The 3000+ figure is from peak to valley, which you'll only ski all of at the end of the day) Great scenery (Mt. Assiniboine is Canada's Matterhorn), and some great terrain with some really gnarly stuff in Delirium Dive and the Wild West. (you'll need an avy transceiver and shovel to get in) Advanced skiiers will want to spend most of their time at Goats Eye, which has a fast quad and lots of great black diamond + terrain.
Lake Louise - About 45-60 minutes from Banff, gets less snow than Sunshine, but retains it really well on the backside. Some serious steeps off of the Summit Platter and in the Ptarmigan area, with some wicked ass bumps in Paradise Bowl. (right below the lift
) The views here are as good as it gets, and make sure you take the trek up to the Chateau Lake Louise to check out the beautiful hotel and postcard views of the lake and Victoria Glacier. Skating on the frozen Lake Louise is a MUST do... my wife and I loved that spot so much we got married outdoors there last March.
If you have any other questions, be sure to let me know. Hopefully mntlion (a member here) can chime in with a locals perspective.