What the difference between a 18 teeth freewheel & a 16 freewheel
Neither is 'better' but a 16t freewheel will make it harder to pedal at low speeds, but you will be able to ride faster and keep up speed when your going faster. You'll probably have to shorten your chain though.
An 18t will be easier to pedal (only slightly) but you won't be able to ride as fast. Won't make too much difference though.