Well, the only thing about a Cali featured AR that applies to the upper is the overall barrel length. Piston or Di systems shouldn't affect your deicision.
The lower on the other hand......
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul