Hey
I did a season in Banff last year. I worked for the year before and did a snowboard (yeah, deal with it) instructor course so I can go out this year and work. I didn't need a VISA as I didn't work out there but speaking from people who have got one, it's a bitch. The VISAs are released in batches and they go real quick- have an account set up before the day so you can get one. Literally they're snapped up within 30 mins. It's this hard as the UK apparently refused Canada a small loan during the recession so they're pissed at us. Having said all of that, my mate was in the low thousands on the waiting list having missed it and he still got one. If you have a confirmed job offer then this helps getting a VISA as well.
I can't speak for Whistler but Banff is soooooo sick. I'm from Birmingham and being in a town where everyone is stoked on snow is pretty pimp. The board shops know their stuff and aren't out to rip you off (ask for a locals discount anywhere and it'll normally get results). None of the residents are really making it rain but everyone's really chill, the nights out are great (the girls are pretty fit generally) and the town in general is just super chill.
The snow is great as well. I was mostly just at Lake Louise for the season and that was fine. The park there is better than Sunshine and Norquay (although night riding at Norquay is sick- it's kinda like nights at a snowdome but with legit kickers). It's pretty snow sure, the powder is great, there's loads of tree skiing and by the end of the season you know loads of sick spots which are pretty much untracked.
So yeah that's pretty much it, feel free to ask/PM me if you want anything else