I've had my HK USC for several years and like it a lot as an HD gun, but it is limited to 10 round factory mags or 20 round Promags. I have one promag that actually works 100%, I treat it like a gold bar. These are extremely accurate out to 50 yards.
I bought a Ruger PC Carbine a while back and love it, mine has a 500 lumen Surefire G2 and a green laser zero'd at 15 yards, I also added a linear compensator and it really does make a difference with muzzle blast indoors. Mine had the Mlok handguard from the factory so adding gadgets is pretty simple. I had a Sig Romeo5 on it but have moved that to my AR15 RCP (rifle caliber pistol-I just made that up I think) and have a Romeo7 arriving for it tomorrow. I've been practicing with it at a local 25 yard indoor range the past few weeks and it's getting to be pretty fast and accurate, certainly at 10 and 25 yards I'm much faster and more accurate than with any of my pistols. I actually like the standard stock also, I've got 4 AR's but this still feels better to me. The only downside for an AR user is the manual of arms is different, but I keep a 27 round Magpul magazine loaded with 147 gr. Bonded Golden Sabers so hope that fast reloading won't be needed in a home defense situation.
And the 10.5" AR pistol, just finished building this last weekend so have only had it to the range for one day. Even with the linear comp on it's still much louder than the PCC indoors, although the muzzle blast is less than a 16" AR with a flash hider on it.
The AR pistol and the Ruger PCC are almost exactly the same weight of 7 pounds 4 ounces (about 2 oz. difference on my scales) with a loaded magazine by the way, although the Ruger is more muzzle heavy. My standard AR Carbine is a M&P Sport II modified with a drop in 13" handguard, linear comp, Surefire G2 light and Strikefire II red dot, it weighs almost 1 1/2 pounds more than either of these. I'll keep practicing with both but right now either one is completely acceptable as a home defense gun to me.