You can't go wrong with either...
The P30 is the more up to date model from Heckler and Koch... Ergonomics are better
However, the USP is a classic design. I own a USP-C. I got the USP for the classic design.
Youll find a USP online for cheaper than a P30 as well.
Both are equal in reliability (super duper reliability) and have more likeness then differences
If I had to do it all over again, I might have gone with the P30 (was unaware the model existed at the time) but if you cant decide there are a handful of other pistols you could look at too. A Sig Sauer P226, Glocks, etc. But HK is an amazing brand. Hope this helps