I'm going to quote from a post I made in 2008. I got a piston gun because they were all the range and much better than the old direct impingement guns:
"I just head-to-head compared a new POF piston upper to a new rock river gas impingement. Guess which on failed to extract within the first 100 rounds, and which one went through 200 rounds of wolf perfectly (my version of a torture test). If you guessed the expensive one jammed, go to the head of the class.
Here's another little piston tidbit. Lots of ink has been spilled on how the piston is superior because it keeps the hot dirty gasses out of the receiver. That is true and the workings do seem to be cleaner. Apparently it is cooler as well, and that is supposed to be good for it. We'll here's the problem- it vents the gasses under the hand guard. As in where your hand is. Apparently, I'm the only person in the country who, when given the option of heating the receiver of his rifle or cooking his support hand thinks that the former is the better choice. I am not sure why other gas piston guns like an M1, AK, or m14 seem to avoid this. But I can tell you from first hand (no pun intended) experience that the piston AR did."
If you want a piston AR because you want one, have fun with it, it is fun to get new toys.
If you want it because it is solving a problem with the direct impingement guns, think long and hard about it. Understand the benefits and limitations of direct impingement vs piston. It wasn't as if Stoner was an idiot who chose DI for no good reason: he looked at the trade-offs and made his decision. Likewise Kalashnikov: he faced a different set of circumstances and made a different but equally informed decision. Is the best answer in YOUR circumstance to get a piston gun? It could be. I decided that in my environment, for my uses, a DI gun is the best choice.
If you get a piston gun, bear in mind that they are proprietary systems made in relatively low numbers and buy whatever spare parts you may need. They are not like a standard AR where there are replacement parts everywhere. I'm not talking about some fantasy SHTF scenario either, I'm talking about the wars ending, defense department procurement for experimental weapons gets cut back and the company that made your rifle solely to attract a military contract decides that it isn't going to happen and then you need a new o-ring on your piston or whatever.