four strokes swing a bigger prop at a lower rpm, causing the prop to be more efficient, quieter, and great for 3d planes. for the same amount of power, they give you about twice the amount of fuel efficiency. they also make a bit less mess than two strokes, and instead of sounding like weed-wacker engines, they sound a bit like the real thing.

two strokes turn faster, are a lot noisier, messier, and a lot less efficient. they are also a lot cheaper, and may be the better choice if your plane is fairly heavy. for instance, there is a comnpany called flying thingz that makes a flying lawnmower. it would be a heavy thing with a four stroke, but flies fine, lightly, and is capable of doing basic loops and rolls with a .50 two stroke. they also will load up when you idle them, while four strokes have a little less strong tendancy to do that.