They definitely both have wicked terrain. On a bright sunny powder day, I will always go to whistler. If its foggy, I will always go to blackcomb because they've got better tree skiing. I generally know blackcomb better, so I can find pockets of pow there a couple days after a dump when everything else is tracked out, but I've been on whistler with some friends who have found some sweet shit there too.
You gotta try out both mountains and see what you like best.