A comparable (application in a given plane) four stroke is going to be about 40-50% larger in displacement than a 2 stroke. Therefore a .40 two stroke should be compared to a .52 - .60 four stroke.

Power: The two stroke is going to have more top end horsepower, the four stroke will have a broader usable power range. In other words if you are inclined to fly a plane at full throttle constantly, you might be better served by the two stroke. If you like to get good response from your plane at varying engine speeds, the four stroke is the way to go.

Fuel consumption: The two stroke pulls fuel ever two strokes, rather than ever four, so it will consume more fuel, on average.

Cost: The four stroke is going to cost you 50-100% more than two stroke.