The size inflatable depends on how many crew you will be hualing around and
the area you have to store it.. Don't just count on towing it all the time..
Won't work in open water or in anything more than a 3 ft chop.

The out board can be sized to the size dingy and the load you will be
hauling around..

I have a 9 ft West Marine inflatable with a rigid bottom.. I don't have deck
room for a RIB and the roll-ups don't perform well and you can't really walk
around in them. They now have a high pressure inflatable bottom that I would
like to try but I'll wait until I get a few more years of service out of
this one..

Oh! I also have an inflatable keel in mine, Actually it's just a long tube
between the rigid floor boards and the boat bottom.. When it is pumped up
firm, it really improves the performance of the boat..

I'm not even sure that they have a 4 stroke in 3hp and it think it would be
heavier than the 2 stroke..

I use a 2.5hp 2 stroke Nissan on mine and since I'm usually in it by myself,
I can get it up on plane if the air chambers and keel are fully inflated.

I can't complain, both the boat and the motor have served me for about 4
seasons plus a year while I lived aboard at an anchorage.

Shop around and ask others at the dock for a test drive in theirs.

Steve
s/v Good Intentions