Let's be honest for a moment.
If you are looking for braggin' rights regarding WOT speed you'll probably favor an OB and maybe a bracketed OB with the cav plate 1.5 inches above the keel.
If your looking for a bit quieter engine noise at a lower less irratating pitch and are willing to give up 12-16 square feet of cockpit space that can double as a seat/"cocktail" table/ casting platform/ baiting station/ sundeck, with better fuel economy and a more "solid" ride, then you're going to favor an I/O. (guess which side I'm on?)
The reality is that this is why they make chocolate and vanilla!
Regarding speed I just did a quick calculation. If the distance is 20 miles and an O/B WOT speed is 43, then you get there in about 28min., if your WOT on an I/O is 39 then 20 miles is about 31min.
My question is: Whose wife has gotten mad at them for being only 3 minutes late getting home from a fishing trip?
Just a thought!