You have the facts you need, and for a bike to run around town on, and frankly either will be fine. If you like pushing and higher tops speeds a little more then go for the 17, otherwise the 18 is better. Thats about all there is to it. And since this is SS you dont need to worry about skip patches and all that.

I know it may seem like a big decision (I do the same thing when considering bike parts), but just get one and if you end up not liking it after a few months change it up. Its not like it cant be replaced for a relatively small amount of money.