i would say not to buy them at all.
there is a growing trend of people jumping straight in with co2 for their first flipper.

I recommend a low pressure system based on a compresor, if for no other reason than to gain experience before investing in a co2 robot. Its also a hell of a lot safer. A very good flipper can be made this way, for a fraction of the cost, and a lot less weight than a co2 system.