a taller tire is slightly more stable, but turns a little slower. a shorter tire is slightly less stable, but turns in quicker. happy medium is usually a 120/65

also, i think you're looking at your tire size wrong. typical sportbike fronts are 120/60, 120/65, or 120/70. i've never seen a 50 or 55 series front tire. that would be freakin SCARY small.