Steel,
From what I know owning a 19 and having friends with them. The top speed is equal but the 18 is quicker. Id say 150-200 pounds difference in weight depending on layup. My 19 is running over the 100 mark and feels like a cadi doing it. As far as top speed, that question would be better answered by the pros. Also, the beam in a 19 is much wider providing for ample room if you are looking for full interior. I'd say if one was looking for a river boat the 19 is the way to go.
Balls