To harphunt's first post. It really depends on what plane you want to power and what kind of flying your wanting to do with it.
I use both 4 and 2 strokes. on my larger planes and aerobatic ones I use 4 strokes and on my smaller lighter fun fly planes I run 2 strokes.
If I need anything over a .90size engine on a plane, it will be a 4 stroke. "Exception- Just put a 91 on a 60 size midget mustang" That is the largest 2 stroke I will ever own.
engines I prefer are OSfx 2 strokes or YS 4 strokes. The power to weight ratio on these engines are excellent! A bit pricey but, if your putting it in a plane you spent three months building and have several hundred dollars in, better use a proven engine. I feel better about it any how.
Maybe that will help you a little.