These PDA cases are all universal but have different sizes, functions, and accessories.  The 2600 is a great case for a PDA user that does not have any expansion needs.  The 1900 is a compact, fully functional rugged case that can be expanded to work for with GPS, bar code readers and almost any other accessory.  The 3600 is a larger PDA case that can accommodate many accessories inside the case itself and with add-on accessories.  The 3600 is also the easiest to customize due to internal PC board mounts and many accessories.