The Glory Might of the XM29.

[Vangelis]

I think the one you're missing is this, from Wikipedia:

* The M320 operates in double action mode, with an ambidextrous safety. In case of misfire, the M320 operator merely has to pull the trigger again. The M203 used a single action mode which was cocked as the breech was closed. The M203 operator has to cycle the breech to re-cock the firing pin, then pull the trigger again.

This seems more than a little awkward, and I've heard it's also possible if you're in a hurry to mis-cycle the M203 and not actually cock it in the process. I guess the question they're asking is whether it's worth buying a shitload of aftermarket parts from KAC which the M203 itself was never designed to accomodate and getting armourers to attach them, when they could just buy a new system which gives them the same capability right out of the box, and is actually designed to have it.