When I stayed in Cottonwood heights I rented a car. But I wanted to do other things in addition to skiing Alta/Bird and Solitude....catch a Utah Jazz game, ski a PC resort and ski Snowbasin. I would think if you are just going to ski the Little/Big Cottonwood resorts the bus is the way to go. Like I said for me I wanted the flexibility.
If you decide to rent a car I was given good advice. Rent both a 4WD and a "regular" (cheaper) car. Then see what the weather will be and cancel one or the other. When I went back in 2007 the weather was forecasted to be fair so I cancelled the SUV at the rental desk and took the cheaper car. Worked for me and no cancellation penalty at all.