The significant difference between a pneumatic system and a hydraulic system if the effective compressability of the working fluid; pneumatic systems using a compressable gas and hydraulic systems using a relatively incompressable liquid.

As there are stated no restrictions on the design, and no requirements for variations in required power, it should be easier to design a more efficient system that uses a relatively incompressable working fluid vs. a compressable one.

Since a compressable fluid is absorbs enegry from the system when it is compressed, the amount of energy converted to heat (and the fraction thereof lost from the system without performing usedful work) is more significamt than with a relatively incompressable fluid (with relatively little energy being wasted in that fashion). This should be the dominant factor, since other friction losses (throttling, piping,etc) are made trivial by the lack of specificty in the op's hypothetical.

benbenben

' I refuse to have a battle of wits with an unarmed person'

Thermodynamically, neither system would match the efficiency of modern power stations using the water/steam system as the energy transfer fluid. Which is sort-of-why it isn't done in those places.

the eficcency in thermal equipment is about 90% or more, in hidraulic equipment is already 80%, but the seleccion depend of energy you have available, cost equipment, easily instalation and other considerations

Some water turbines are over 90% efficient. Steam turbines around 40%, and gas turbines--higher temp & pressure-- 60%. Compressed air is more efficient then thermal, but maybe not as much as water.