The only benefit really would be the ability to use the same mags as your handgun uses. Personally I'll stick with my AR-15 loaded with 31 rounds of SP .223 than a gimmicky rifle loaded with 16 or so rounds of JHP 9mm.
I'm pretty sure that the sub2000 can take the 33 round G18 mags as well, so you can potentially get pretty good ammo capacity out of it. I wouldn't think that 9mm is going to penetrate any worse than .223. You can always choose a lower pressure cartridge if it's an issue. The advantage over a handgun would be that you should be able to shoot faster and more accurately with 3 anchor points than you can with one. Sort of a poor mans mp5.
But either way, the OP has the carbine and may or may not have an AR.
And, I think - assuming maneuverability of a long gun in general isn't an issue - the keltec should work quite well for home defense.