Car rental definitely not needed at Aspen with free hotel shuttle to airport (waiting when you land) and free shutttle bus route between Aspen & Snowmass. Very convenient - easier than Whistler (where the airport is 2 hours away) or even Park City (45m to Airport).
As for skiing plenty of easy blues and blacks at Snowmass. Those in the group needing a challenge can bus over to Ajax. It’s all much better than Whistler which often suffers from lack of snow or icy slopes