We looked at all the shuttles, and while they all seemed adequate, ultimately we opted for the rental car. Having the option to go to Waikiki, the North Shore, etc. whenever we wanted was great, and even though we didn't use the car that much, we were glad we had it the times we did.
Remember, Aulani is on the southwest corner of Oahu. You're not going to get very good bus service out of there, and it's not really walking distance to anywhere interesting (save maybe the Paradise Cove luau). The amount you'd pay for busses or shuttles would probably be better spent on a car.
That said, there is a rental car desk at Aulani (Alamo, I think), so if you get a shuttle and then realize you want to go somewhere else, you can still do that.