Sorry I did'nt see your post sooner, its right up my alley. We have built plenty of both, and although equally as exiting to drive I believe the I/O to be the better boat. There are many attribute to the outboard. Such as cockpit space and fuel capacity, However the need for three motors to get any real speed kills the maintanance and reliability.
The I/O has a better center of gravity with the weight down low in the bilges. Will definetly run harder in a rough sea, and be somewhat easier to maintain in the long run.
Fuel consumption on the outboard of the same speed will eat gas in comarison with the I/O. You can put more girls on an outboard though ! I would guess you need to really think of the 70% use of the boat to make a decision. Either boat rocks.