The main difference is that the size 120/65/17 is the perfect medium between tires... the point of getting bigger or smaller tires is the main feeling of turning... say youo trade the stock 120/65/17 for a 120/70/17, the bike will pull out of a turn better, but it won't turn into the turn as well... same as if i were to trade down from a 120/65/17 to a 120/55/17, i would be able to enter a turn better, but i wouldn't be able to exit it as well as i could with a stock size of 120/65/17... but then again if your not a real hard twisty rider and mainly use the bike for commuting, you won't feel a big difference, heck you may not even notice any difference at all... but changing tire sizes in the middle of a track day, you'll DEFINATELY feel som change... so yea, stock 120/65/17 is a perfect tire size, but if you don't really care to much about turning, and don't turn to hard, just settle for the 120/70/17...
kow wee