It's more about how you like to ride. The steering angle will change w/ tire diameter, and placement so I think of it as how you would like the bike to feel. Bigger tires are generally a softer feeling ride, and vice versa. Then theres style... If you allways feel like you are falling over the bars go w/ (2) bigger tires the same size. If you want a quicker feeling in the front go smaller on the front, and bigger on the rear.