Piston AR-15/M4's have become a popular trend in recent years as a supposeded "fix" to the shortcomings to the problems of the M16/AR's Direct Impingement gas system.When the M16 was introduce in Vietnam, it was claimed to be "self-cleaning", but that statement turned out to be absolutely false since the AR platforms gas system is considered to be very dirty & requires heavy maintenance to make it reliable since it deposits it's gas system residue in the chamber.
The problem with the new piston driven AR's is that the AR was designed by Eugene Stoner around the Direct Impingement gas system. He wanted to make a very light rifle and to do this, he eliminated the operating rod that was the norm during the time and actually still is as the AR is the only platform I'm aware of that uses D.I. The AR had a very fast cycling rate and is known for great accuracy since there is less reciprocating parts to add to recoil forces, but at the expense of running hot and becoming unreliable if dirty or in unfavorable environments.
A friend of mine recently had a conversation with an Army Ranger who didn't think the piston AR's are a good idea because the op- rod could torque the carrier and cause fracture failure of the lower receiver near the buffer and many others have backed up this assertion. He did go on to say that this probably wouldn't occur for thousands of rounds or on full auto, so neither of which is a real concern to the average Civilian AR owner. Still, the DI AR system has been refined as much as it probably can be to the point that it is a fairly reliable system and has used by the Military for over 40 years with good results even though they have been looking into replacing the M16 with a piston system.
I think if a piston system is desired it's probably best to opt for a plaform that was purpose built to be a piston rifle from the ground up, not a conversion. The piston AR's are simply not proven as of yet and not as common or parts as available as the standard DI's. So, for now, if you want an AR, I'd stick with the standard Direct Impingement.