Hi, Chunkypuffs!
I've just bumped up a thread on scrying where I went into different types of crystal balls and others had lots of useful info too. I've also mentioned a book that I found useful. I notice some other threads on crystals in the divination section if you call up everything.
One factor will be what you can afford. Basically lead (ie reconstituted) crystal is the cheapest. Obviously size of ball affects price tag. I've got three lead crystals - small, medium and large. To be honest, the medium one is probably the one I use most - the small one is too small to place on a stand and see easily and the large one is likely to pick up lots of reflections which aren't present in the other two. I've also got an egg shaped one with lots of inclusions, which is wonderful for relaxing with: under electric light the inclusions are silver and give off rainbows and in candlelight it glows golden. I also have an almost black stone (apparently original scrying mirrors were painted black to remove reflections) with wonderful rainbow shades in it - can't remember what sort of stone it is, but it's good because I can hold it in my hand to scry with or mediate with the colours in daylight.
It might help if you give an idea of what you have in mind. For instance, if you want something you can carry in your bag and pull out to meditate on wherever you are, you'd be better off with the smallest - crystal is proportionately quite heavy!