I'm going to go with Luke being faster and stringer than Ventress, and perhaps more skilled. I'd say their Force powers at this point are relatively undeveloped but still rather powerful. I'd still say Luke is superior though, considering he has the Skywalker penultimate force sensativity. Ventress was supposed to be the slight superior of Anakin as of AOTC.
This would be a good duel, but I see ROTJ Luke pulling through as the clear superior in this fight. Of course, a more recent version of Ventress would take this duel, undoubtedly Luke takes it here.