I personally think you can't go wrong with the USP, but the new HK45 is nice too. I would say it depends on the price difference.
I think Glock makes great guns too, but they aren't on the same "level" as H&Ks. I say this because I own a USPc40 and a Glock 19. I like the USPc40 better overall, but then again the Glock is lighter weight and less bulky. But in your case with the fatter and bulkier Glock 21, that point doesn't apply to your decision anyhow.
I think the H&K is better built and shoots better, plain and simple. If the $300 price difference doesn't matter to you, go H&K.