We'll let you know when testing is done:

outboard by a long shot, I have had both on a 21' daytona, my 300x would beat the blown 502. In heavy boats I don't know the outcome buy if you are talking twins I would assume the same results. Not to metion you can have a jack plate and adjust your X on the fly. If you where to set up the IO to a higher X you will never get on plane.

power to weight. as well as length and balance. a 24 skater with 2.5 race out boards v.s a 28 skater same power. 28 is faster and is longer and weighs more, however the 24 accelerates much harder.
a 36 skater triple race 2.5's v.s a 36 with 525's. roughly same power, same length, different c.g, slightly heavier is the i/o. but it is faster, and accelerates better.
i would say depending on the application, purpose, and design that smaller needs the outboard will win, for larger needs the i/o has it.

The reason my last 2 boats have been outboards are that I can get into shallow water better with outboards, you can run many more hours before a rebuild...a lot more hours, maintanance is less with an outboard.

Pure speed...I'd go with an I/O.

These boats are the same hull, same total hp, same weight and same boating water.

-The acceleration of the O/B is much faster.
-turning of the I/O is much better
-fuel consumption (both 87 oct) is about the same.
-Speed is very close, the I/O is proving to be a little faster, 93 compared to 90.
This makes sense if all is the same except for the drag of the second gearcase.

Conclusion;
The O/B is more lively, youthful, ego on the water.
The I/O is more stately, mature, ego at the dock.

not crazy about the paint on the o/b, but either way , nice pieces. hot.

[QUOTE=VetteLT193;2276083]Generally speaking, that 350 mag / 300 HP will outlast the outboard by a long shot. More cubes per HP generally means longer life. QUOTE]

I'm not sure I'd agree with that. In an average use scenario, I would have to think a 10 year old outboard with 600+ hours (I've got that) would outlast and need less repairs than an I/O given the same use, operating conditions, and care. Manifolds, boots, and corrosion issues are the first things that come to mind. Not to mention, it's been said over and over that OB's are designed to be water borne propulsion units, and the I/O is a land based engine modified for water use. Remember, I'm a salt water boater..........

36 skater with 3 2.5 280's = 840Hp and 119mph
36 skater with 525's = 1050Hp and will not go 119!

In a smaller lighter boat, like 24 superboat, the OB will be faster. The OB can also accomodate a larger cockpit.
The OB would have less maintenance.

screaming obs do sound pretty cool.
and its not a pair of 350 stock sbcs sounding as good.
remember that!