Aside from the physical dimensions & size of mounting bolts, the main differences are in the material used & their ability to withstand vibration, shock & corrosion.
Basic brackets are made of steel & have basic shock & vibration resistance.
Cast brackets have a far greater vibration & shock resilience. They are also more corrosion resistant.
The best performing brackets in terms of resistance to vibration & shock are our patented Quake Damped brackets available for the Scorpius range of lamps.