The M-11/9 is to the Class 3 world what the AR-15 is to the semi-rifle world...
It's the most versatile and affordable (that's the only thing that knocks the M16 out - the damn price) MG you can get, in terms of uppers, stocks, etc...
Every time you look, there's a new upper, or part out... Although the ATF has played fast-and-loose with the rules when folks try to make belt-fed uppers for them, some DO exist in circulation (I seem to remember a RPD upper - it cost more than the MAC itself)...
The only advantage the M10 has over the 11 is that if you're a .45ACP fan, the M10 fires said caliber...
Although I wouldn't put it past someone to make a M-3 grease-gun upper for the M-11... After all, they made a Soumi 9mm upper to convert the M-11A1 from 380 to 9mm (said upper features a separate magwell, takes drums & sticks)...