Saitos: definitely messy.

YS: A little less messy than the Saitos and the most powerful of the bunch.

OS: Very clean, not quite as clean as a gasoline engine but not bad. Very little oil on the plane after a flight. They run very good too. Probably second only to YS in my opinion.

2 strokes: Nothing wrong with them at all. Definitely cost-effective. Just gotten to where I can't stand listening to them. Inever say never though. I may have one again for something one of these days.

All just my opinion of course, your experience/perception may vary.

Bottom line is fly what you like and have fun.