Unless you have more than 3 people, the Mears shuttle is the way to go. The cost is about $19 pp, r/t. It takes, maybe, 15 mins longer than a car service would. I've used it several times...we didn't make a ton of other stops.
I like to get the first bus over..usually around 7:25, which gets me to US/IoA by 8. The last time I took this bus, I was able to get to RPR, get checked in, and then over to IoA by 8:30, so got in early with the other resort guests.
But, if you have a group of 4 or so, it may be worth it to hire a car service. Those seem to be running about $75-80 or so, plus tip.