You must analyze where you ride, your weight, trike weight and your conditioning.. If your area is hilly, the 20" wheel giving lower gears would be advantageous. If flat, you might want bigger for more speed. If you face many intersections and traffic controls the smaller wheel will give better acceleration, which is handy for stop signs. I have owned several recumbent trikes, and the best one, gearing wise, for city and mixed riding was an 81 speed Greenspeed with 16" wheels. I always had the right gear for any situation, and could accelerate away from a stop sign quickly.