Re: 2 stroke or 4 stroke?
More info behind your question would be good - how big a boat are you looking to power, how much $$$, etc.
Generally, I think it is fair to say 4 strokes or direct injected 2 strokes (i.e. E-Tec) are taking over, and 4 strokes now dominate new sales in the smaller size motors. Old tech 2 strokes are already banned in some places, and I would expect bans to gradually spread, particularly in "green" states like Cali.
At pretty much any size, a 2 stroke will be cheaper to buy, and lighter - maybe 30% lighter in some cases.
4 strokes have a rep for idling better, and therefore work better for trolling while fishing. Around here they seem to be particularly taking over amongst fisherman on the sound for that reason.
Personally, I just bought a new 20hp Tohatsu 4 stroke, and because I plan on using the engine the rest of my life, didn't even consider a 2 stroke.
-dm