I have just called both Mammoth and Bachelor on this very topic, as I was not taking for granted full operation past April 15 due to low snowpack at Mammoth and bean counting at Bachelor.
Mammoth intends to run Canyon Lodge and full hours of operation to April 22. I would note that March is not starting out favorably, and if it turns out to be as dry as January the whole Canyon side of the mountain could be very sketchy on snow by then. The Main Lodge area will run into May this year (closing time typically around 2PM) even with no more new snow.
Bachelor is loaded with snow, but will not commit to full operation past April 15. They will look at their business in early April, and if it seems strong they might go another week of full op. When Bachelor cuts back for spring, only Pine Marten and Summit operate (still serving about half the terrain on the full vertical), and the lifts close at 1PM.
Given the above, I would go to Whistler if you are committed to that week. It's also the week of the World Ski and Snowboard Festival. Hopefully that does not make lodging limited or expensive. Banff will have some good skiing, but with its lesser snowpack Goat's Eye at Sunshine and the off-piste on the front side of Louise could be rocky. The Whistler/Blackcomb alpine snowpack is going to last a long time this season.