How much more do you like the 18 then the 17 or do you like the 17 better? If your anything like me and play 5-8 hours a day during the summer you would be restrining 3-5 frames every 2-3 days. Which I would go with the 17 because string racquets would take to much of time that I could be spending on court.
Do you string your own racquets? If you do and you don't play that often go with the 18 if you like it better. If not the 17 is the way to go.
My strokes- http://www.youtube.com/watch?v=JrhYnuZFn3M
My serves- http://www.youtube.com/watch?v=gW3SE7fS9vU&feature=plcp