Harris isn't very good, but I think Ellis for Rondo is probably the most equal player for player swap you could make. Both probably have similar trade values at this point.
I don't think Rondo would necessarily fit on the Warriors, though -- the reason they want to trade Ellis is so Curry can play PG full time. It could make sense if it were part of a larger deal in which the Warriors get, say, Granger, and the Celtics get Ellis.