I've got the Beretta CX4 and Ruger PC Carbine in 9mm, and a Hi Point in .45. The Hi Point is cheaply made, but that's part of the appeal. It's also a bit heavy, but with accessory mag holders you can have 27 rounds of .45 onboard, which is pretty nice. I'd like something nicer in .45 (hello Ruger?), but I like it. We sometimes have problem bears where I am. While I'd never hunt bear with a .45 ACP, I think it would do better in a defensive situation than 9mm.
The Beretta is very short and has a feel of quality, despite its space-age styling. I didn't like the look when I saw pictures, but holding it completely changed my mind, and I have no regrets at all. The plastic wings that protect the sights cut down on some peripheral vision, so that's a negative. I'm going to eventually upgrade some of the internals with Sierra Papa parts. They are expensive, but I have no plan to sell the Beretta so I will take the plunge when I've got some extra cash.
The Ruger is much longer as well as heavier than the Beretta, but obviously, it breaks down easily. I got the model with the barrel shroud, mostly on a whim as it was the same price as the non-shrouded version, but lists for more. I was shooting it in cool weather a couple days ago without gloves (low 30s) and the shroud was a little cold. I think the nylon-stocked version might have been a bit more comfortable in that circumstance, but it wasn't a big deal.
It's hard to say which 9mm I like more, because they are so different. The Ruger has a longer sight radius and the weight helps it steady a bit easier, while the CX4 handles more like a subgun. I'd recommend checking both out if possible.
For home defense I am sold on the pistol-caliber carbine. The noise issue is the primary one for me. I have had a number of emergency situations happen when predators have suddenly appeared coming after my livestock. There's no time to put in ear protection, and I value my hearing, thank you. Crosshair has it right in his comment. I don't have tinnitus, but I know people who do, and I want to do what I can to prevent it.
There is a tendency among gun enthusiasts to value power above all else, but there are a lot of other variables in shooting. .44 Mag eclipsed .44 Special, for example, because people wanted moah powah! But the Special is just as good as it ever was, just not as popular.
Pistol caliber carbines are fun to shoot, cheap to shoot, and the right tool for certain jobs, like home defense.