Keystone has a small portion of the front mountain open for night skiing. It is groomers only and no advanced runs.
I've had a good time night skiing there, but usually it is about the people I am with more than it being a good skiing experience.
With night skiing, imagine what a high-traffic groomer feels like at 4:00 on a busy day. Then, take all of those groomers and freeze them rock solid. Add in enough light to kind of but not really see, make it cold as hell, and you have Keystone night skiing.
If you are going to fit two, It would make sense for it to be Key and A-Basin- but the better Keystone terrain is all in the back mountains, so if you are going someplace else in the afternoon, it will feel like you just got where you wanted to go and now have to head back. Especially because lines at North Peak and Outback can suck.
Regarding A-basin, it is the best mountain in Summit County and it makes no sense to ski anywhere else.