Glock 19 20.99 oz, with 15-30 rounds in factory high cap mags. 17 rounds can be had with a simple +2 mag extension, with spares holding 19 rounds the same way.
HK USP 9mm 23.52 oz. I believe (but could be wrong) that it holds 13 rounds, period. There is no +2, there is no mag for a larger model that will fit.
In my opinion the Glock 19 is the only way to go if you're going 9mm. The only real point of 9mm is to carry as many rounds as possible, and with 124 grain +P it can be some pretty hot shit. With a little practice you can learn to really dump multiple rounds into a target extremely fast, and with the low cost of 9mm ammo you can afford this learning.
I don't dislike the USP, it's just that in 9mm or .40 I don't see the point. Now in .45, that's another story.