Since you are already using a DI rifle, do you have problems keeping it clean/maintaining it?
Personally, I would go with what shared the most parts with something that I already had.
That being said, even if you do get a piston upper, and you later find out you don't want it or the supplier doesn't support it anymore, you can just swap to a new DI upper (or whatever the flavor of the month is at that time).
One of the great advantages of the AR family of weapons. The lower is the "gun".
I have heard really good things about the Sig 516 though. Supposedly one of the lead engineers from the HK416 program (the premier piston upper) designed the Sig 516 and added in some interesting improvments.
I am no longer participating in gun forums.