Yeah this situation is completely hypothetical, but really that's only because in real life you're never going to have such a definite line of choice between girls and skiing.
As for me, I think I'd go for powder. Balls deep in the most perfect snow is something you (or me anyways) can talk, dream, or reminisce about for the rest of your life. It's usually a longer experience too. There are so many girls near the top for attractiveness to go after any time.
I think the only reason though that I'm thinking pow is that I got to hike to some boot deep backcountry stuff yesterday and I have pow on the brain and am hungry for something deeper. At the end of ski season once I've got bunches of powder days logged it would almost certainly be the girl.