For a true AR it's probably best to stick with direct impingement. It's what they were designed for and it works just fine. Piston retrofits have their own set of issues (like carrier tilt), so it's not like they're definitively better.
If you truly want something piston operated, then I would buy something that was designed to be piston operated from the get go. An AK, an FN SCAR, a Bushmaster ACR, ect.
Most of the problems people have with modern DI ARs can be traced back to one of three things: bad operator, bad magazines, bad quality AR. If you buy a good rifle (Colt, BCM, Noveske, LMT, Daniel Defense, ect), run good magazines (like PMAGs), and learn how to run and maintain the firearm properly....your problem level should be at a minimum.
Another thing to keep in mind is that carbine length gas systems are the most violent in operation and therefore put the most stress on components. If you are looking for the best possible reliability in the long term, a rifle length gas system might be the way to go.
Another option that is starting to become more popular is the mid-length gas system. This seems to be the best compromise, as you get a longer gas tube, but still maintain the short maneuverable barrel length of a typical non-NFA carbine (16"). I don't own one yet, but for my next AR I'll definitely be looking at a mid-length setup.