I currently ride an 07 Large Stumpjumper FSR. I'm going to buy an Orange 5 frame. Upon demoing an 18" 5, it did feel a little cramped but it did have a short stem with the saddle in a middle position on the rails.
I'm 6 foot 2 and looking at the geometry the Effective Top Tube Length of my Stumpy is nearest to the 18" 5 than the 20".
How tall are you and what size 5's do you ride??
Its worth noting that i have a 65mm stem om my stumpy so could easily go to a 90mm stem on an 18" 5. I know i should really demo a 20" but i much prefer the look of the 18".