Poppa is not too far off there; however, I would still go Twins, especially if you are planning to run offshore. I have come in on a “kicker” to many times!
Whaler? I might know something there.
If you go to Whaler.com: I believe Whaler quit offering 2 strokes. That should tell you something?
http://www.whaler.com/boats/boat-250outrage/
To compare fuel consumption, you have to start with same boat weight, then same RPM, or same MPH/Knots. If you compare a single 300 versus twin 150’s on the “Outrage 250” you will find addition weight, but the two props do give a little better performance. It really isn’t enough to make a difference one way or the other, at least with a “Verado”, which is what Whaler uses. It will diffidently affect the cost.
Single 300 – 4000RPM -24.7 MPH – 11.1 GPH, WOT 6300 RPM -43 MPH – 30 GPH
Twin 150 – 4000 RPM -25.5 MPH – 11.3 GPH, WOT 6250 RPM – 45 MPH – 31 GPH
Kicker, I’ll go with the extra weight and cost of the twins, rather than the extra weight and hassles of the kicker. I haven’t used a kicker for over 10 years. One less thing to worry about, and maintain. I really enjoy not messing with a kicker!