If you have to change it I would suggest the 18". It offers a better tire selection with a taller sidewall and you can even go to a trials tire, which is not offered in 19". As hughes said the 19" was designed primarily for track to keep the sidewall short. I haven't had any flat issues with the 19" and I ride in sharp rock, but with an 18" I might get away with a lower pressure. Cam.