We used the Allegiant option of transfer last year for the two of us, and we were transported in a small van to our Disney hotel, and then they picked us up right on time. There were 8 people on the van going to hotels, but only the two of us returning. Since we landed at 11pm, and were only staying a few days, it was worth it to not have to drive.
We have rented cars several times. I always check all of the rental car options, and try priceline and expedia too.
Another option, but may not be any cheaper, is one way from Sanford, turn car in at Car Center or Dolphin, then one way back.