9mm in 124-grain through a 16" barrel (such as the Hi-Point) was the only 9mm round still gaining velocity when it left the barrel.
Don't quote me, either, but I think it would have more to do with the powder than the bullet grain. Slower is almost always better when we're talking longer barrels. The thing is, almost all 9mm powders are very quick burning so few 9mm rounds gain much from the last 4" or so of barrel on the Hi-point. Either way, it's still doing more than it would out of a pistol.
To the OP (if his mind isn't made up, this thread is old), I would go with the Hi-point. It's not that I love the carbine... I really don't. I would prefer a 10/22 all day for plinking, hunting, fun, etc. You're asking about home defense. A 10/22 would be reliable if it weren't for the ammo feeding it. I don't trust any brand of .22 ammo to be 100% (or even 99%) reliable.