The Shock Factory stuff uses two concentric tubes with a bladder between them that takes up the variation in volume as the piston rod moves in and out, so no need to charge with nitrogen. Because the shock doesn't run at high pressure the seals get an easier time and there's less stiction than you get with a nitrogen charged shock.
The single adjustable varies compression & rebound damping in a fixed 3:1 ratio, but the double adjustable has completely independent adjusters. On the home page of my website there is a link to a video taken at the factory showing a shock under test. You can see as each adjuster is moved only the compression or rebound line moves, the other is completely static confirming the independence of each adjuster.
We have a hydraulic preload adjuster in the prototype stage, but it won't be ready till into the new year and we still can't commit to when. However when its released it will be retrofittable, so you could go for the shock now and put the preload adjuster on when its released. Having said that there is plenty of room around it to swing a C spanner.
Cheers, Mark