14 tooth and 15 tooth... what's the difference compared to stock 16 tooth??  any difference between 14 and 15?

14 tooth, not recommended, unpractical.  Easy to do wheelies(too easy?), mega torque but you'll be running too high of rpms at cruising speeds.

15 tooth, good mod, popular mod.  Faster get up and go.  Effects high end performance.  The speeds you could once attain an lower rpms will be slightly higher.

One tooth down on the front is equal to three up in the back.