Breck and keystone are both pretty dope. Both great parks. Breck has better jumps and rails, Keystone has better creative features.
As far as all mountain, breck has better steeps and keystone has better groomers. Overall i think breck takes the cake.
I'd say stay in breck, 'cause the town of breck is a lot better, and ski at both if you have a rental car. keystone is only 25 min away. Vail is about an hour away but it's def worth the drive for the back bowls.
definetly vail if your going with your family. that mountain is so big you wont even notice if you ski the same trail twice. the only line is for the gondola because everyone is so spread out on the mountain. if you want to skip the line at the gondola, take the chair right next to it, ski to the right when you get off, and take the next chair up. you'll end up at the same place.
also maybe go to beaver creek a day or 2 if you're staying there the whole week
^i missed the part about going during spring break. i guess there's gonna be some lines then