I researched this subject for quite some time about 18 months ago. I decided to go with a Power Mover AC-8.
Power Movers | MOVING MADE EASY
I've been extremely happy with it. I would do it again too.
Depending upon your specific application you may find that the ball or jack mounted dollies are not very safe. In some instances the trailer can apparently tip over the dolly or "spit" it out. I didn't want to be holding onto the dolly if that ever happened. If you have a totally level surface this is less of a concern. The Power Mover bolts up to the frame of the trailer to prevent this type of accident. However, it is still quick and easy to connect and disconnect the unit from your trailer.