The Diorite builds on the ground breaking technology tested by the Flint. While the Diorite uses the same ADS control system and reactive armor concept , There are several upgrades which allow the Diorite to operate for longer duration that the short range Flint.
The Diorite includes an oxygen garden to supply the vehicle with fresh atmosphere. A standard module is also added that can be used to transport cargo, or replaced with a med bay depending on the mission profile.
The interior wasn't the only thing to get an upgrade. The propulsion system now has a reverse LK Lander Core engine to aid in deceleration for orbital insertion. The extra hardware and interior space make the Diorite well suited for medium duration missions.
Update: Here is a list of all the vehicles in the armored vehicles series : Flint , Diorite, Granite , Mica Schist.