I don't know about head to head testing that is fair to both sides. Hopefully someone will be along soon with a few links.
you know that old bit of advice of not buying a car model in its first year of production? Well the standard AR has been in use since the early 60s and by now all of its issues are known quantities. We know when to replace each part. We know what wears and how. What parts break and how. This is super valuable real life data based on hundreds of thousands of rifles in use all over the world.
Pat Rogers has had several articles on his loaner ARs that are used in his carbine courses. He has several at over 25000 rounds and one that is at 42000+ rounds with no cleaning just lube.
http://www.ar15.com/archive/topic.html?b=3&f=118&t=526041
There are numerous articles flogging piston guns for a reason. They are the flavor of the week and the companies that make them want to sell them. Gun magazines love to write about the latest new thing. It helps them sell magazines and advertising.
I bought a BCM middie and a POF piston rifle at about the same time. shot side by side both are very nice. Both feel different from each other with the BCM being slightly smoother and the POF slightly more accurate.
However at 1700 rounds or so on both rifles, the POF, due to carrier tilt wear at the mouth of the receiver extension, suddenly allowed the buffer retainer to come loose tying up the gun. The BCM at the same round count showed the EXPECTED slight wear on all internal mating surfaces.
The POF now has a special extended receiver extension and a special ball bearing roller cam pin (yes, MORE moving parts) that should, in theory prevent this in the future. Will it? Will it cause other problems? What problems might crop up? who knows, its an experimental system at this point. Come see me in 40 years when it's as grown up as the standard AR.
When POF, or any of the piston AR producers go out of business getting spares for that proprietary gas system will be no fun!
Best quality parts for standard ARs are ubiquitous and the variety endless.
If you want to go high tech, and build a next generation AR you don't put in a piston. Build your AR with the latest coatings on the bolt and bolt carrier, use a fluted nitrated hammer forged barrel instead of chrome lined, try a monolithic upper and throw a Geissele trigger in it.