its all been said, both are about the same distance (breck is maybe 20 minutes farther) and come on the same pass. if you are thinking about moving to summit county, dillon is pretty much equally between them, or move to one and make occasional trips to the other.
breck seems like it is more of a resort though, with a better night life, and the crowds that come with that, although even last weekend wasnt that bad.
keystone has night skiing and slightly smaller crowds, and an equally sick park, but from the time ive spent there, maybe not as much of a night life