I went Whistler Dec 15 last year as an intermediate boarder and had a fantastic time. The high alpine was open, but only it hadn't been open really long. Whistler is top tier in North America, in terms of terrain and the village scene.
I'd go as late in December as you can but before Xmas and it's really a bargain. Early december is pushing it but mid december or later is a great value and still pretty good snow.
Whistler has amazing deals on if you book by October 15, you can stay for example at the fairmont for I think 199 per night and can that bundled with a cheap flight. The fairmont is a great location, ski in/ski out, pools, bar service while you sit in the hot tub, great atmosphere, and so on.
For a first trip for your girlfriend out west I'd say go whistler (and strongly recommend the fairmont at current prices) unless you're hunting perfect snow conditions in which case you'd sacrifice the various other benefits of Whistler for the snow.