Hello Guy,
There are basically two types of mover, A-frame, and axle movers.
The A frame movers in essence replace the jockey wheel, either by clamping into the jockey wheel bracket or coupling to the hitch. That have just about the same ability of pulling or pushing that an adult can achieve, so if you can't handle the caravan by your self, then there is a major chance that an A frame mover will also fail.
If you do not have any slopes or bumps to negotiate then they are good system, but if you intend to take them with you, you have to be able to clamp/unclam them and finde somewhere to carry them, and of course if the site you want to use has any slopes or bumps, then you are no better off. You must also consider the effect of rain or other loose or slimy surfaces that will decrease their effectiveness further.
Axle movers have a number of clear advantages of over A frame movers. Primarily the amount of grip generated is much more, so they can stop and hold a caravan on a much steeper incline than an A frame mover. Their grip will still be much greater even in poor surface conditions, and this is also aided by the larger diameter wheels being easier to climb small bumps etc.
Axle movers are also permanently available at home or on site. Their major down side is the extra weight they use up from your payload capacity and purchase cost.
Just a note, most axle movers should climb about a 1in 4 slope. If you slope is greater, then you may need to consider a winch to pull the caravan up and let it down in a controlled manner.