I am looking to buy an airbrish machine but really know nothing about them....is there one that is better than the rest?
and does it have to be specifically for cake decorating?

you don't really need a cake airbrush, I have a testor compressor and a kopykake airbrush.

This thread should help you decide http://www.cakecentral.com/modules.php?name=Forums&file=viewtopic&t=608279&postdays=0&postorder=asc&&start=0