Four stroke definately....They burn alot of feul aswell, but have more torque than two strokes, and the big prop swinging slowly causes drag and slows you down on the downlines. Most two strokes tend to rev up and down lines, so you go faster. Also, the latest YS four strokes ara as powerfull as similar sized two strokes. However, they are expensive, and if you damage it, it can be expensive to fix.
2 Strokes are simpler, and have the same, if not a little more power as four strokes. They don't have as much low end torque though.
I would go with the YS 140 DZ. It is said to run very nicely, smoothely, with little vibration, and matches the OS 140.
Robert.

My 2C!!!
Robert.