Breck is a bigger better mountain with better parks, more variety, steeper and more difficult terrain. It is soooooo crowded if you are skiing on the weekends. Breck is likely to be more windy than keystone on the same day.
Keystone is smaller, still has a great park that is pretty creative and conducive to progression, it is still a pretty big mountain, but doesn't have the bowls and steeps that breck has. A lot fewer crowds even on the weekends, but during peak weekends still gets quite crowded, but still better than breck. Night skiing is soo fun even after the park closes is you are a decent skier and can rip groomers and side hits.
Keystone is closer to Denver by 15-30 minutes depending on how fast you drive. The vibe at key is better than breck IMO. Breck is more appealing to skiers who like steeps and bumps or need bigger park features. Keystone is more appealing to people who are mellower, like all of the park in one place, like side hits, and big low angle bowls on powder days.
I don't ski much park so I don't have much to say about Breck's because they are kind of spread out unlike keystone's where the whole park is in one place so I like to ski park at keystone more.
Try them both and see what you like
