The USP has HK's patented recoil spring, which IMO, tends to make it a softer shooter. However, the bore on the USP is higher in relation to the grip which tends to make muzzle flip more pronounced. I haven't ever shot them side by side to see of it makes any practical difference.
Having shot both at different times, I'd say there was no remarkable difference in recoil or muzzle flip; but that the stock USP trigger was much better (assuming you can reach it given the size of the USP grip).