I use the AC6 from http://www.powermoverinc.net/
and have been very happy. Pros: it is a 1 person job storing and hitching the boat, I have complete control and and can place the boat in the garage with no problem, and I can turn the trailer in almost 360. Cons: tweaks the tires on the rim as it rotates, sometimes the motor doesn't stop right away and you have to plan for a slight coast to stop, there is a limit to the height of a lip it will travel over, and it will only travel on a gradual slope.
hope this helps.