There are two types of mover typically available: A frame types, and axle types. A frame types typically have to be fitted each time you want to use it, which means finding somewhere to carry it if you want to take it with on holiday, and as most carry their own battery they can be quite heavy to manhandle.The axle mounted versions are permanently fitted and are available at any point of your holiday or storage.
The major concern with the a frame devises is their ability to grip the surface enough to control the caravan when on slopes or on loose or slippy surfaces (e.g.wet leaves and gravel etc). most of them have plenty of power but its no use if there is not enough grip, and probably more importantly is the ability of the mover to stop the caravan and hold it safely on an incline. On the plus side, the do not use up much caravan pay load, unless you carry it in the caravan. These are great on near flat surfaces and are quite effective under those conditions
Axle mounted systems have plenty of grip, and they will stop and hold a caravan on most practical slopes. The down side is the weight of the mover being permanently fitted to the caravan. All the axle mounted units use the caravan existing battery for power.
There are some manufacturers who have built up an enviable reputation for after care service, Excellent as that is, other manufactures may have less problems, and thus their after care services are not needed so often, the difficulty is getting hold of the necessary information to see which if any manufacture or model is better than the rest.