You might want to check out this thread I did awhile back comparing the USP 9mm and the G17 with two other guns:
http://www.canadiangunnutz.com/forum/showthread.php?t=144500
I have the G17 and the USP. I like both and both will continue to make many trips to the range for years to come.
I think that some of the above advice is good: decide what
kind of gun you want then decide what
brand you want. A simple pull the trigger and that's it-type gun, or a more "tech"-type gun with SA/DA, decocker, safety, recoil reduction system, different variants, etc. The first category would be things like Glock, XD, Steyr M-A1, S&W M&P, Walther P99QA, SIGs with the DAK trigger, etc. Second category would things like the SA/DA USP's, P22X SIG's with SA/DA Walther P99AS, and most of the guns you see in the shops by Beretta, CZ, IMI "Baby Eagle", etc.
HK's gun that competes with the Glock 19, would be more like the P2000 LEM, which unfortunately is not available to us in Canada due to barrel length.
If you handle both guns you are interested in, you will find the USP to have a much larger feel to it than the G19. The G19 is 6.85 inch long and 5 inch high, whereas the USP is 7.64 long and 5.35 high... so almost an inch longer... and an inch is huge in handguns. The USP is also somewhat wider than a Glock and sits higher in your hand.
G19's are not as soft shooting as G17's, and USP 9mm's are very soft shooting due to the recoil reduction system, so that is a difference that you may want to consider.
Everything with handguns is personal preferance. I would get the USP though. I love that gun. Mine hasn't stayed home a single weekend since I got it, whereas my other handguns sort of rotate through a cycle of range duty. And taking something to the range and shooting most of your 9mm rounds through it every weekend, that is probably the biggest compliment you can give to a handgun design.