Advantages and disadvantages of 48x18 vs. 48x17?

TheBrick(Tommy) 17 it's prime and prime numbers are the building blocks of all other numbers and hence really really cool. You should base your decision solely on this fact alone. Do not be swayed by how easy / hard it is to pedal.

On that basis run a 47 on the front and 17 on the back. Works out about 74.6GI