I went to Aspen/Snowmass last month for a convention. All I can say is wow. Highlands does look cool and I'd ski there. Snowmass looks like the Sugarloaf of the West. That is one HUGE mother mountain. I really want to ski there....
Buttermilk--meh. Maybe worth a day.
Aspen reminded me of a bigger Cranmore. Interesting steep terrain. But I have no idea where one parks to ski Aspen.
I skied at Aspen last year TB and you are right. Highlands is where it is at. So cool. and if Highland bowl is open you should definitely hike that. Snow mass is massive and good on a powder day. I think there are day lots in town and buses. Snowmass has parking right at the hill.