The USP is the older style basically... I recently sold my Hk45 and opted to keep my USP 45. I like the feel of the 2x4 though... The Hk45 and the P30 were designed to feel better in people's hands. They are more oval in shape like a 1911 with finger gooves. I like the way the USP shoots also...
Personally, I would say to get an exact replica of your gun in an airsoft gun if you can, and train with it. Some might say that is a joke, but you can run tons of drills, and practice clearing your house. Some of the higher powered guns will punch through cardboard that you can make silhouettes...
It depends on why you want the newest date... It sounds like you think that the newer it is, the better it is..... That is not the case with most guns. Look at Hk as an example. My USP from 95', is the same as my USP I bought new two years ago with the small difference of the catch and hammer...