One of the advantages of the AR system is that the lower is the "gun".
If you get a piston upper, and you later can't get parts or decide you don't like it, you can always switch to a DGI by just swapping uppers.
That being said, the pistons will always be more expensive than the DGI for a small reduction in cleaning time. I guess it depends on how valueable your personal time is.
Now if you are planning on coming up out of the water like a SEAL and shooting immediately or going with really short barrels and firing full auto, then a piston has some advantages over DGI in those situations.
I am no longer participating in gun forums.