Before investing in power for my new Gulfstream, I asked the service guys..they said the 4-strokes rarely come back for service, but the 2-strokes are more prone to problems.
I can't speak in terms of longevity, as my 200 4-strokes only have 180-odd hrs. on them. "0" problems so far..excellent engines, but many on here have over 1000 trouble-free hours on theirs.
There will always be a 4-stroke VS. 2-stroke debate, just like Merc VS.Yamaha, but the general concensus here is that they are very reliable.