As you know there is no way to forecast the snow so the "safe" answer you are looking for is not possible. What I can share is tha living within 3 hours of Fernie I can attest that Fernie also has rain problems and it's rain line is usually much higher up its mountain. While Whistler's base is at a lower elevation than Fernie, Whistler's summit is nearly 1200 feet higher and offers lots of skiing off of its summit. So, if it rains where do you have a better chance of getting in some skiing? Don't get me wrong, Fernie is great, but when it comes to spending big money to travel from New York I'd go to Whistler.
Besides skiing, if conditions are to be poor, I'd rather be in Whistler as it has a much more vibrant village. Again, while I like Fernie it is limited in restaurants, shopping, nightlife, especially when compared with Whistler.
Since you are an expert skier have you considered heli or cat skiing?