Having done it myself, try lugging an M3 and a few 30 rd. mags around. Heavy!
The 9mm subguns have merit, but those that are reliable are hard to get and costly, e.g., the MP-5. The clones are north of 2 grand. Many only come in open-bolt, like the Swedish K configuration which isn't too pure in a household setting. Or are full auto only. The Camp Carbines were pretty good, but they aren't being produced any longer. Many reported reliability issues.
The modern M-4 (or M4orgery) is as small and light as most 9mm subguns and fires the .223. No contest. It's replacing most subguns in most applications, including home defense.
If you want to shoot...shoot...don't talk! Tuco
USAF Munitions 1969-1992