Like chubb, I've tried both on many bikes over the years, and I've always had a strong preference for the stock seat. It's more 'solid' yet softer and more pliant (read comfortable!). The foam seat is harder and less easy to slide your bum over. It's also often less 'fixed' or solid, despite being harder, although this comes down to how solidly you've mounted your tail. Also, getting to the battery/quickshifter/power commander is muuuuch easier with the stock seat!! All my racebikes have stock seats.

If anyone wants to, Superbike Source can supply just a stock-seat race fairing tail on it's own, without buying the whole kit, for most makes and models. PM me if interested. Most bikes will cost $160 shipped to you for members (retail is more).