I've been to Winter Park and Keystone. Winter Park is by far my favorite, but I've been there many more times and seem to know the lay of the land a little better. Keystone was alright and I enjoyed the night skiing, but I like Winter Park as it is a little less crowded and seems to offer plenty to do (both on the mountain and in the town). Though the town is not huge and doesn't have tons and tons of stuff like Breckinridge, it does have options and seems to be less tourist and more local. Wherever it is you go, enjoy the trip, it will be a lot of fun.