M4 stock is an AR style, they are the same thing (if you are refering to collapsable). As for what you should get, that depends on how much money you have. As I'm a poor student I am currently running a 4 position Rock River Arms collapsable stock. If you have the money and feel like it, you could slap a Magpul M93 or other high end stock on there.
Personally, I had Josh make me a custom adapter that raises the stock and puts my M4 style stock inline with the barrel. This won't work with the average rifle as the iron sights are too low.