I would say to try a bunch back to back and compare on the launch monitor. I went to the shop that had a Cleveland fitter there for free a couple nights ago and tried a few clubs and shafts. When I was hitting stiff shafts the ball flight was a little inconsistent. As soon as I switched to X flex, it was much better. My buddy, whose swing speed was a mile an hour or two faster than mine, was just the opposite. The stiff flex shafts were all that were working for him. Different swings, tempo, loading of the shaft, release, etc. all add different elements to how the shaft works. I think it all ends up being a bit of trial and error.