Most of the 2 stroke 10hp and 15hp models are the same weight, possibly the 4 strokes too, which by the sounds of things isn't really a major factor for you (unless your removing it each time).
Without turning it into a 2 V 4 stroke debate ill add some "thinkers" for you
Your fuel supply. 2 stroke will mean carrying a spare pre-mixed tank. 4 stroke can offer the option of swapping connectors on main tank
Storage. If its remaining on the transom not so much of an issue. If you intend fitting when needed a 4 stroke will need careful consideration for storage. Plus they are heavy to hang over the transom in a swell.
Performance/economy. Depends on the distance it's likely to need to take you. If we're talking miles you'll need to be carrying. Larger spare tank for 2 stroke. 4 stroke again you can use you usual fuel (which obviously doesn't help in event of main nine failure due to fuel contamination) supply.
Purchase/Service costs. 2 strokes are not really much cheaper to buy right now. They will be cheaper to service. Re-sale value will most likely remain with the 2 stroke in the short term, 4 stroke long.
Going back to the subject of what size, I think an 8hp is your lowest consideration and 15hp your max. Anything less will struggle for a long haul, anything more is overkill.
A 6hp will give you around 5/6 knots without taking into consideration conditions. Sounds fine for a short trip back in flat calm seas but sods law tells us it'll be in a chop you need to use it.
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