I got the 14" rack as I thought the larger one looked too big, especially if you weren't riding with side cases. I just needed something to hold my cold weather jacket because this time of year in GA it's been below 40 in the morning on the way to work and then above 60 on the way home and the factory saddle bags are a little too small for my jacket. The 14" rack, a couple of Rok straps and I am good to go. If you are a long haul / cross country rider go with the 18" but if you just need a little more carrying capacity the 14" works well.
Thinking of buying one of these racks as soon as I get the new bike home. Feedback would be appreciated, 14 or 18? Did you buy one of the other and wished you had the other one?