Little Cottonwood Canyon - Alta/Snowbird - Great terrain and snow
Big Cottonwood Canyon - Solitude, Brighton - Not as severe terrain as Altabird, but also great snow
Park City= The Canyons, Park City, Deer Valley - Mellower terrain and lower (not as geat snow)
Layton - Snowbasin, Powder Mountain. Snowbasin is just great fun, PowMow, not so much, but it's big.
I wouldn't stay at Altabird since there's so much diversity you'll lose by not hitting the other hills, and with the number of people you'll have, you'll want to keep the opportunities open. I usually stay in Kimball Junction which gets you to any of the non PC areas within about 45 minutes, and the PC areas are up the street.
Breckenridge - my absolute last choice of the bunch due to its terminally intermediate (my terminally intermediate friend called it beginner) terrain.
A-Basin - the gnar of the I-70 areas. Great vibe. Everyone needs to go there at least once.
Vail (aka Flail) - will remind you of WB for size (it's only 5000 acres vs WB 8100 but it meanders everywhere), another somewhat marginally challenging place, but there's something for everyone except if you're going to be in the next Matchstick movie.
Beaver Creek - high end ambiance and good terrain
Keystone - doesn't get much respect but I really like the trees and the snowcat terrain on the Outback side.
Steamboat -1.5 hours away, but worth trying for the trees everyone's heard about. Intermediate style hill, but great snow, and the trees MUST be skiied to be believed.
Copper - Went once and didn't like it, but I think that was the company and the fact it was the 8th or 8 days. Gets a lot miore respect that Keystone - go figure.
If you stay in Breck you can get to the other areas fairly quickly.
Squaw - fairly legendary terrain, with great potential for Sierra Cement
Heavenly - some challenge but you sort of have to look.
Plenty of other areas around.
Good chance for a lot of bluebird days.
Asspen is great - 4 areas in close proximity, but it's a pain to get to, and pretty pricy.
If I had 60 people, it would be SLC and staying in the Kimball Junction area, or CO, I-70 in the Breck region. If I HAD to make a decision - SLC - no brainer.