I have both , a Nissan 6hp 4 stroke , a Johnson 5hp 2 stroke , and a Cruise & Carry 2 stroke .
The Nissan is brand new , the Johnson 3 years old and the C&C is ancient.
The 2 stroke will use twice as much gas or more and at low RPM's discharge unburned oil into the sea , 2 stroke fact of life . It is more powerful and louder for the same HP although lighter , nuther 2 stroke fact of life .
No mixing gas and oil for the 4 stroke , so if you top off a tank , you can forget the higher math and if you get it wrong on a 2 stroke , you foul the plug or burn the rings out . They are not inviornmentally friendly untill you get to the higher HP models with 'oil injection' .
All 2 strokes smell , I dont notice my Nissan .
Also I can talk to someone on the bow with the Nissan running hard , can't even hear myself with the Johnson .
I get 5knots in flat calm water with the wrong prop on the Nissan 6hp ( new prop will be here Friday ).
Which ever you get will need a 'power' prop of about 6" pitch , 9x9 is standard and wont give full power or rpm's , unless you buy a designated 'saildrive' , 'sailmaster' ,etc. which will have the right prop .
Yes to the alternator option , it aint much but will light the running lights or top off a battery that is low ( not enough for Airing's microwave ) .
Which do I prefer?
I like both , my wife only likes the Nissan and has very ugly things to say about the Cruise and Carry ( poor thing only weighs 12lbs and is asked to push over 5000lbs of boat).