You know what this thread has made me realize.... Almost all the companies producing piston guns put out top quality rigs. I can't really think of many factory piston guns I wouldn't own, LWRC, Hk, POF, LMT, PWS, and even Ruger; the majority of the manufactures are really good.
Now that being said, I also have to acknowledge that there are a lot of companies producing fantastic DI guns, but, there are just as many if not more producing sub-par equipment. This is a very general statement, but it seems that if you get a piston gun, no matter how you slice it you're getting something worth owning. Think about it, if you were to walk into your local shop and ask for a piston AR, chances are your only choice will be something nice (and expensive); if you were to ask for a regular AR, I'm sure you would have plenty of DPMS, Olympic, and Bushmaster options...
Just a wild assumption, but it seems that the extra cost and complexity keeps the jokers from playing the piston game. And to answer the OP's question, they're all good, it comes down to personal preference.