Depends on what you'll be using the engine for.

Four strokes are more fuel efficient. They have half the horse power of two strokes of the same displacement but have much more torque. Therefore, they are ideal for scale planes and 3D flying where you want to "hang on the prop". More torque allows four strokes to swing bigger props.

Two strokes have more horse power and turn at a higher RPM. If you want speed, a two stroke is what you want. Generally, two strokes are lower maintenance and easier to set up. They're idea for any application really. It's a jack of all trades engine.