I've been on vacations to both and I would go 100% with Whistler.
People complaining about rain in January at Whistler is straight false. Yea it'll rain all day at the base but that means its snowing up top which is where all the good skiing is.
Jacksons sick but its too small and can get boring.
Whistler is the greatest ski resort in North America. There is more than enough of every single type of terrain (trees, open bowls, sunny faces, glaciers, steep rock chutes, gnarly wind-lip faces, natural jumps, groomers, sick park) like it legit has every type of skiing imaginable.
Jackson has some good expert stuff but Whistler is way crazier, tons and tons of gnarly chutes and legit double blacks, like there are in-bound lines that will make you shit your pants just looking at.
Also Whistler has amazing lift access to get to anywhere you like with little to no hiking (10 minutes tops seriously). The mountain has amazing flow and each part of the mountain feels so unique.
There's a site called whistler 3d solutions I think, google it, if you cant find it, pm me, but it uses google maps and shows all the great runs so you can see exactly what they're like and then go ski it. Its the best guide available for any mountain and its free. You'll find everything your looking for and more by using that.