It is possible to hop around on the back wheel of a bmx, it is just a hell of a lot harder. I would say that 20" would be better to start off on as they are so easy to control, whereas 24" are an aquired taste if you catch my drift. As for having a go, come to london on saturday, there will be a couple of us riding and it should be a good experience, bring your bmx and have a laugh!