You know, maybe I'll just stick with the buses, no rental car at all? Have you guys ridden them? I'd be going midweek - M-Th, so less crowded than the weekends, I'd expect? Do I just get on with my board, boots, and gear already on?
You shouldn't have any problems going mid-week on the ski bus. You just get on with your gear already on, they have racks on the bus for your board. If you're only wanting to check out the SLC resorts (Brighton, Solitude, and Snowbird) you could pass on renting a car and use only the bus.
We did that last year for our trip. We stayed at the Super 8 in Midvale. There's a couple restaurants, a grocery store, a Sonic, and some other stuff within very close walking distance. The ski bus stops at the corner, its about half of a block from the Super 8. You can get a SLC Super Pass at the front desk, which is $56 a day. It covers your lift ticket to any of the 3 resorts, plus your ski bus fare there and back. You just show your pass at the ticket window at the resort and they give you a lift ticket. You don't have to pick what resort you're going to ahead of time, so you could buy a 4 day pass, board at each of the 3 your first 3 days, and then go back to your favorite the last day. It was perfect for us, and after going to a few Colorado resorts the year before, Utah was much better.
Just make sure you know what time the last ski bus leaves from the resort. Some of them don't run past the time the resort shuts down the lifts, so you do need to keep an eye on time.