Given your main concern of using the widest rim and tire you can, then 17" should make that easier than a taller rim of 18", given a tire of the same height.
I think Jay's pics say it all...definitely 18's. More room for brakes as well which is never a bad thing. Assuming similar tire heights, wheel diameter doesn't really play a role in determining your max width for fitment purposes as the outside profile of the tire remains the same.