This has been beat

, but here's my humble opinion anyway:
Pistons
PROS: run cooler and cleaner, and are especially good for sustained full auto fire and suppressor use (think HK416). Last I checked, none of us do too much of that...
CONS: they're heavier, and each manufacturer makes them differently. Parts aren't universal, and they're more expensive and even less available
DI
PROS: Mil-spec is mil-spec... Parts are easy to find (most of the time!) and drop right in.
CONS: they run dirtier, but I let mine go 1000 rounds or more between full cleanings (usually pull the bore through after range sessions, that's it), and have no issues with it running smoothly.
Bottom line: 99% of military guns are DI. That's good enough for me!
I suggest you consider mid-length or rifle gas systems rather than carbine. IME they get a little smoother the longer they are.