On second thought... I've been looking at Bike Bandit's tire selection. If you'll look down the list at both 17 and 18 inch rear tires on separate pages for available sizes, you'll see that much of the increased availability for 17 inchers seems to apply specifically to the crotch rocket crowd with their wide rubber. (Or whoever else would use that on that size rim...)
The 18 inch tires are more suited to the narrower sizes, dirtbikes and classics expecially. But on the other hand, you have to factor out the non-street legal dirtbike tires in the case of the 18 inch.
One thing to keep in mind is that there doesn't seem to be a good selection of 17 inchers below 120 mm width, while 18 inch tires have a decent number of options down in the 110 mm region.
Not ready to make the call yet on which size is best... but... for those comfortable using 120/80. 120/90, and 130/80's, 17 inch is a good choice. If you want the option of going narrower, go with 18.
You don't wanna get the wrong setup and feel like you have to use front tires on the rear because all the rear tires you want are too fat.
Pages: 
