A Sig 556 is a better choice than both an op-rod AR or XCR. Look into the 556's before you make a purchase. There is alot to like about them...
Piston AR's are a gimmick in the civilian world and IMO, many of the currently availible models aren't "quite right". There are many issues to tackle when retrofitting a weapon and changing its entire operating system. Retrofitting a weapon, as opposed to designing a new one, requires you to work within a certain set of perameters that may or may not be conducive to your idea.
Obviously, some are better than others. The POF seems to be good, as does LWRC however, neither had the development expertise or budget of the HK416. The HK416's prevalence among the most elite of the American military suggests its an excellent weapon. With its release up coming, I personally wouldn't buy a piston upper until the HK is released.
Even still, piston uppers don't offer much on a 16 inch AR carbine. IMO, certainly not enough benefits to justify the cost and "leap of faith". For better or worse, DI's are a known quanitity. Its well known that if you have a good AR and you take care of it, it won't ever let you down. And I don't mean cleaning it every 10 rounds. That's not what AR's need to work. They need good components and "stewardship". Buy a nice weapon, use good ammo/magazines and lube it appropirately and it won't fail you.
If you aren't running a sub 14.5 inch barrel on a F/A lower with a can, you won't ever see the true benefits of an op-rod system.
IMO, the best availible civilian piston gun is the Sig 556. With the newer models, it just keeps getting better and better.