I rarely use either. If I'm specifically talking about the holiday that falls on Dec. 25, in informal correspondence (e-mail or on message boards), I'll say XMas. Generally, I just say "Season's Greetings" because I have too many friends and family members who aren't Christian, and I don't feel they should be left out of warm holiday wishes.
Besides, trying to keep up with all the 'appropriate' greetings for every denomination (or lack thereof) gets tiring!
