I have done this many times (WDW to US to WDW) - car rentals, mears, limo's and taxi's. I spoke with many CM's about this and different concierge staff at many resorts and the majority of them, as well as I, believe a taxi is most logical method of transportation
Mears and Linx charge per person ($12-$14 / person) and you have to be ready to arrive/depart on their schedule. You can probably find a deal online for a 1 day car rental under $50 but then you're restricted to being back before 6pm and you still have to get transportation to and from the rental agency (swam/dolphin/grovsners all have agencies in their hotels) plus then you have pay parking fees while at US/IOA.
A taxi, IMHO, is the best method to go from WDW to US. The cost is about $25 each way (depending on how much you tip) - so $50 roundtrip. You will be dropped off at the esculator by the parks entrance from the parking garage. When departing the parks, go to the same spot you were dropped and will see a large convoy of taxi's waiting to take you back.