Are both motors running carbs or fuel injection? Fuel injection gives 10-20% better fuel economy on the same motor. However, despite the also design differences in the Johnson and the Yammie, they will run about the same fuel economy when properly rigged if both have carbs or FI.
Let's put some things in perspective, cuz everyone talks fuel economy, but few actiually "get it." Best cruise on your boat will be about 25 MPH - that will probably be about 11-12 gallons per hour on a 2 stroke carb motor, or about 2.2 MPG. Now, let's say you go from a carb motor to a direct injected motor, and you get a big improvement of say 30% - wow - that's a lot, right? Well, your fuel flow would go from 11.5 to 8, but mileage only goes from 2.2 to 3.1. You are saving at most 2.5 gallons per hour, or about $7.50 per hour at cruise speed. So to offset the $6,000 difference in price of the 225HP motors between DI and carb, you will need to operate the motors for 800 hours to break even. Most folks never get their motors to 800 hours unless they are using them commercially.
IMHO, either motor would work fine. Pick the one that is most likely to get you home every trip and go with it. Also consider one other thing - electrical output. Some older motors used the stator to charge the battery, whereas more recent motors use a automotive-type alternator to charge. BIG difference! The stator-type system can only put out about 10 amps when everything is working right, but the alternator system puts out about 60 amps. With all the pumps, lights, electronics (radar, anyone?) and other gear on today's boats, you NEED that extra charging power. Again - apples vs. apples - no real difference if both the Johnson and the Yamaha have alternator-based charging systems.
See ya on the water!
