Being the owner of a Glock 21 and an USP .45, I have to say since you are making this a home defense gun, I would recommend the USP.
You can easily add a tac-light to the USP, which makes it a great night firearm. For the grip, I would add a hogue rubber sleeve to it, makes mine much nicer.
Now...PERSONALY I resonate with my Glock 21 more than my USP, and I intend to get a 30 as well. But when I am around the house, I have a Glock 22 on me. When I'm asleep, I have my Mossberg 590 next to the bed.
On the debate of the mag release, I personaly find the H&K's to be a sure way to constantly pinch the hell out of your finger.
Another thing to think about is that you can use the Glock 21 full-capacity mags in your G 30, giving it 13 + 1 cartridges.
I'm sure none of that was of any help on clearing things up.