Own the Keltec, my best friend owns the HiPoint, so I've shot both a good deal. The HiPoint is ugly as sin, there's no way around that. It is also amazingly accurate for the price, outshooting my Keltec by a small margin. It feeds almost anything without a hiccup, while my Sub 2K just doesn't like certain ammo, Remington UMC for instance. Recoil is sharper with the Keltec, but that's because it's lighter. Neither gun kicks much, or can't be held on target while emptying the mag.
As for uses for a 9mm carbine, I bought mine as a companion to my XD9 pistol. Doesn't replace a good rifle, but with modified Beretta 92 mags I can share ammo and mags cutting down on what I carry when traveling light/moving fast, and gives me a longer effective reach than with pistol alone. The Sub will put all 15 shots within a 6 inch circle at 50 yards as fast as I pull the trigger (better with well aimed shots), which is better than I can do unsupported with a pistol, and can stow in a briefcase, in a backpack, or under a windbreaker. As I said before, the HiPoint's slightly more accurate (maybe 10-20% smaller group at that distance) but uglier and bulkier.