It depends on when you're going and what your looking for. Dont go to Breck for spring break unless you enjoy huge lift lines and if you are coming to summit some other time, go to keystone for rails and Breck for jumps.
keystone for sure. i just came back from summit county went to keystone 3 days and Breck for one, and honestly i wish I'd gone to keystone all 4. the jumps in Breck are amazing for sure but the lift lines and overall vibe just don't make it worth it. keystone is pretty much empty on the other hand especially in park since its not at the base like Breck and has its own park lift. i got 24 park laps in one day and i got up there at 11, parks open till 6 so you don't need to worry about getting up super early (unless you wanna hit the jump lines which close at like 3ish). the overall flow of the park is so much sicker then Breck cause there's just features everywhere and it seems like everything has a tranny to hit all the features in as many ways as you can think of. keystone is by far the sickest park I've ever hit.
i apologize for the shitty spelling and grammar