I've gone back and forth on selling my USP too.. now that I have the P99s it just doesn't see much use. But I haven't been able to part with it yet.
That said, now that you're getting into long guns, why not? If you're not using the USP, why not put it towards something you will use?
Of course that's the rational answer
Mostly though, I view my collection as an evolving journey.. trying out new guns I've been interested in, learning from it, and moving on - as opposed to simply amassing a bigger and bigger collection for the sake of having more toys..