How many people are you coming with? For two weeks, look at renting a condo with a kitchen and other amenities (ownerdirect.com vbro.com travelocity.com etc are places to look). It can be cheaper than a hotel room for periods that long, and it is much more comfortable than a room with beds.
If you are strong skiers, I wouldn't spend the money on heli-skiing, the day trips you will do out of Whistler are going to be low-angle glacier runs. The excitement of flying in a heli would be cool, but the chance of being in a group with gapers is pretty high. Cat skiing is a decent option, but if the snow is good at Powder Mountain, it's probably as good/better at the resort, so you may as well just ski there. Snowcats.ca at Hurley Pass near Pemberton would be a better option for cat skiing, and less chance of gapers since Powder Mountain does sales out of the village now.
Whether you ski powder or not will depend on the weather. It's unusual for it not to snow in a 2 week span, but it does happen (this December it did not snow for 3 weeks). You can see the weather stats on the WB webpage, January and March are usually bigger snow months than February. The park is obviously world class, and there is a chair on the BC park so you can lap it pretty fast.
Vancouver is a beautiful city, and worth spending time in, however it rains a decent amount in the winter which makes it less enjoyable as a tourist. Also if it is raining in Van it is snowing in Whistler, and you will be pissed you are not up there skiing.