It depends on how flexible they are and it they fit properly. In very general terms short boots and chaps give a better feel for the edge of the saddle and how much you have your leg wrapped around the horse. The cheap rubber long boots are quite bulky and because they tend to hold your ankle in a fixed position can make your lower leg less secure. But if you have a problem with driving your heels down so pushing your legs forward rubber long boots can balance that for a while. Although even then it isn't a great idea long term as you learn to rely on the boot instead of correcting your position.
A well fitted pair of more expensive leather long boots provide the same advantages of close feel to short boots and chaps -and will be less bulky at the ankle. But for a beginner short boots and chaps are probably going to do a better job and be more cost effective.
