it's true. if you're looking at sheer numbers, Whistler is hard to beat. compared to Big Sky... well, I've only done the drive by of Big Sky in the summer, but based on my knowledge of being a nerd, I can tell you that Whistler has way more slackcountry and multi-day touring options right outside it's door than Big Sky. In bounds, Whistler wins, as the do in the park as well. Whistler is under 2 hours from a major city/airport, Big Sky has Bozeman - hardly a "major city" by any means.
Compared to Whistler the only things I would say Big Sky has going for it would be cheeper living costs, lighter snow (although less of it), and less crowds - which can be said for pretty much everywhere except for maybe Breck and Vail.
That being said, Whistler is a good place to visit for the weekend, but I would never live there (unless I win the lottery or something).