Re: suzumar 3.1m RIB, 15hp 2stroke or 15hp 4stroke?
Either design will provide the rated horsepower, but unless you are a mechanic be careful of some of the comparisons people give.
If you do not mind the weight, the four stroke is a very nice engine. Be aware however that repairs require much more than a two stroke due to the additional systems required to make them function, and there are more moving parts to wear. In the 15 HP class, the difference in gas mileage between the two is not as much of a problem as with larger engines. Keeping a two stroke in new condition is much easier and less expensive than with the four stroke. A marine dealer who is one of the best outboard mechanics I know told me when four strokes became popular that the owners would be very surprised when the engines required repairs. In addition to the work required to repair a two stroke they would have to pay for additional repairs due to the induction systems and other parts.
The two strokes we have owned only required one or two pulls on the starter to start. As they have fewer cubic inches for the horsepower, compression releases are not required on the smaller blocks.
For portability, the two stroke is less of a problem as it does not have an oil sump to be concerned with. As you are looking at a RIB rather than a SIB, you may not have occasion to transport the engine lying on its side. If you trailer the rig, the four stroke would serve you well.
You will find that a two stroke may be problematic to find for sale, but don't overlook one if it becomes available. What is best for you is your decision.
Remember that the four stroke outboard came of age due to environmental decisions, not improvements in performance.