Sunshine is where you want to go. It's the best place for early and late season (November & May).
Like you said, at Louise you get the 'Wiwaxy 500'... one run of blown icy snow with a million Albertans on it. Now and then they open with great coverage but it's quite rare.
When Sunshine opens they usually have all the village lifts running (Angel, Strawberry, Standish, Wawa) and a bunch of powder. Even Standish alone is super fun and underrated.
Nakiska isn't worth driving all the way from Vernon for (any time of the year). Pretty much the same goes for Marmot. Banff & Marmot are both about 4-hrs from Edmonton. Castle is the best hill in Alberta, but they have a shorter season so usually not so good early & late - make sure to hit them up later in the season! Sunshine is planning to open on Nov.10 this year... they usually open on a weekday which means awesome pow and just locals, but with a Saturday opening planned expect it to be slammed with the WW's.