It gets really cold at night the surface gets much harder at Keystone. If you are not use to icy conditions I would be mindful of it. But if thats not an issue, riding Keystone at night is so much fun!
I would suggest Breckenridge, because the green and blue runs are very wide and long. And you can take the bus to ride Keystone at night with the same lift ticket if you didn't get enough riding in that day.