Jacob, so obviously, there is the color difference, but there are some other differences.
R3 5900 would come on our R3 Frame. The R3 Frame utilizes a flat frame sheet and has a little more padding in the belt. The belt also has 2" webbing on for attaching accessories. The bag also has some additional internal attachment points for accessories and several reinforced stitching areas required by the government. The pack is designed for military use but still well suited on the mountain.
The Sky 5900 will come on our Xcurve Frame. The Xcurve utilizes a flat frame sheet. The belt is slightly stiffer and has 2, 1" webbing straps which allow you to cup the bet to you hips. The back of the Sky 5900 also has Xpac material, which prevents blood from leaching into the bag when hauling meat in the load-shelf. The Xcurve Frame is often the choice for guide and outfitters hauling numerous heavy loads in a season.
Both packs have the same load rating (150lbs), a 2,500+ cubic inch expandable load-shelf, full panel zip for access to the main bag, are hydration-compatible, and made in the USA.
If you need more information, feel free to call the shop, 406-404-0641.
Cheers-
Lyle