I heard rumor that abit may be coming out with a Slot 1 i815 board - but it's only a rumor.
Anyway, I would go with a FC-PGA and use it on a slocket for older slot1 board.
But now, all i815 is only socket so you really can't get a converter...
BTW s370 chips are getting cheaper and cheaper while Slot1 only dropped slightly!
Performance wise, don't think there IS any difference.. but I can't tell.. it's just differnet packing..