United Kup has shiny paint, while Generations Kup has a matte finish. Also, United Scourge has more paint apps, specifically his blue stripe goes all the way across the wing on the front part of his vehicle mode on the United one. That's basically it for the difference of paint apps. They're not very vast changes like Tracks, G2 Prime, or Jazz.
I have both Generations Scourge and Kup. Both are excellent figures and I feel satisfied with them that I don't need to buy a United version of either of them. Though I am getting the iGear head for Kup. That cygar...
I've also heard of people getting the United Scourge to use as Scourge and multiple Generations ones to use as the Sweeps.