We've got the same SIB, normally run a 8hp yam 2/ with two adults and it's fine, but have put a 15 yam on the back an it transforms it.
I don't know what your experience is and your intended area of operation, but depending on where you are using it (river or sea?) i wouldn't want to be going crazy fast with 4 "people" onboard a 3.2m boat. By the time you've filled it up with all the required kit there won't be much room for moving about
To be honest if i was starting from new id go for the bigger engine, to give me more flexibility but that's based on my experience and use of the boat.
Remember the newer 4 strokes will be much heavier per hp, if you can find a nearly new 15hp 2 stroke I'd go with that.....
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