The p30 and the USP are equals in reliability and durability. The only thing in that field that I didn't like about my P30 was the fact that the grip panel on the back just didn't feel secure. I actually like that the USP doesn't have interchangeable ones...but seeing as how just about every polymer gun out now has them I would say they are g2g.
The other knock on the p30 is the trigger...but if you are shooting it as good or better than the usp than it should be a moot factor.
Short answer...they both do the same thing and are comparable in size. While my heart tells me to tell you to not sell the USP, as mine is my favorite center fire pistol to shoot by a large margin, if yours has taken a backseat to your p30, and there is something else that you would rather spend the money on than having it living in your safe, go for it.
I just tried to like the p30, and couldn't