The USP has proprietary accessory rails (might need an adapter if you will be clamping stuff to your pistol), whereas the P30 has the non-proprietary rails and will accept most all clamp-on accessories without an adapter. The P30 has interchangeable grip panels to suit more hand sizes, while the USP does not.
The USP seems to me to be even more "overbuilt" than the P30, although both are outstanding. The USP has the more robust dual-spring recoil system, while the P30 has the single-spring+nylon-buffer recoil system like the compacts. The USP has the much more aggressive grip texture which I prefer.
Of course there are other differences between them too. Considering the price difference, I personally would choose the USP based upon my likes and needs, but either pistol is a definite winner in my book. ;)