Hi Adrian, First and foremost to consider is cost, What you need to factor in in this decision is, Where is it going to be used?,
Is Noise a factor? ( Am I going to off ps the significant other or Neighbour).
can the compressor be enclosed? (this can lessen the noise, but needs ventilation and access to service the compressor, IE you need to get at it to drain the tank occaisonally).
Generally the bigger the tank/ more CFM the motor is capable of generating means the more expensive it is, so like everything, a happy compromise needs to be reached.

I'd love the space/ cash for a 200ltr tank, but I just don't have it, so at the moment, I'm playing with a 50ltr Aldi Jobby, which for its purpose ( pressure casting resin) is spot on.
Yes its noisy, but it lives box in my shed, out the way of the other half, so jobs a gud un.
(Roll on Finishing the Garage and bigger compressor here I come, Just Don't tell my missus)
as for Duty cycle, thats dependant on the amount of flow from the nozzle, 20 psi = plenty for blowing small debris, and the size of the aperture in the nozzle, so yeah smaller nozzle, more concentrated "jet" of air.

Last question, I dont see why you could'nt put a solenoid on the airline to control it switching on/off. Assuming that a "Coolant output switch" is a signal input.
if it is , then instead of coolant it would be controlling air flow.
most "tank" compressors have pressure switched duty cycles anyway, so when the tank reaches its desired pressure the motor switches off, placing a soleniod valve in the output airline controlled by the coolant input would have the desired effect, it just means you'd have to turn off the mains for the compressor when your done.

if that makes sense.
Rick.


EDIT ,
JAZZ Beat me to it.