I agree tulsamal.
ARs have had decades to mature. There is a plethora of aftermarket parts so you can customize to your hearts content. It is easy to switch uppers, sights, triggers, rails, stocks, grips, etc.
If I was gonna get a piston gun I think I would buy an LMT or a LWRC.
http://www.lewismachine.net/images/p...PS16_large.jpg
This is a pretty good review by PalmerB
http://www.m4carbine.net/archive/index.php/t-4528.html