a) pnuematic- air operated actuator. The plant needs a reliable source of compressed air, usually a with min pressure of 80 psig at the inlet to the valve positioner. For a large plant with many air actuated devices, a central air compressor with drier and filter plus distribution piping is more economical than the laternatives of electric motor actuator or electro-hydraulic actuator.
The air actuated valve must be desinged for relatively low valve stem load, as the power from a air actuator is limited to relatively low levels.
Air is non toxic and nonflamable and does not generate a spark that could cause a fire in areas with possible flammable gases.
b) electro-hydraulic-EH- the valve actuator uses high pressure hydraulic valve  liquid ( at 1500-3500 psig)to drive a piston than will move the valve actuator. The forces that can be developed are much larger ( 25-50 times larger)than with an air actuator, and the speed of action can be much faster. Typical stroke periods of 30 seconds with a large air actuated valve can be alternatively stoked at a stroke speed of as quickly as 0.2 seconds with elector hydraulic actuators. Hysterisis is much lower, and valve position accuracy, resolution  and repeatability is much improved over air actuator.
The cost is much higher with EH . If severa EH valves are used, then one central EH pump with accumulator is used, with a distribution and drain piping system to power the valves. Traditional hydraulic fluid is flammable, which leads to serious fire safety hazards to be dealt with . there are now available several non-toxic and nonflammable sythnetic hydraulic fluids that can be used in place of traditional hydraulic fluid. If only one EH valve is requried, then one can use a single purpose EH pump, and also water-based hydraulic fluid can be used to lower the cost.

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