Wreckman is right. Each engine has it's pros and cons for certain applications. I use a 2 stroke if I want high revs and high speeds. I use 4 strokes when I want lots of torque, quiet operation, and slower speeds (Mainly aerobatic planse). If I build anything larger than your standard 100 size aircraft my first tought would be to go gas. Sometimes there are compromises and you'll go with something you normally wouldn't, but generally I follow those guidelines.

A lot of times I'll stick with 2 strokes because they are more economical, but if I was rich, I'd probably put a 4 stroke in almost everything. I much prefer slower aerobatic planes than I do those speed demons.