14GR seems to work the best at keeping the bike in the peak torque range. As mentioned the 15GR will lower rpm and make a slightly better cruiser. I doubt they would change much for top speed at WOT on a flat road.
That being said the 14GR is the more popular option but I will admit it would be nice for a lower cruising RPM. If you are a heavier rider, and/or like riding 2 up, then the 14's are a no brainer.