Anything would be better than the rudimentary 'whistle' that the tractor comes with, but I chose to install a real, live 'temp gauge'.
I want to be able to see any potential over-heating events
coming, not react to it after it has happened.
A radiator cap with a temp read out would certainly give you some idea of coolant temps under various conditions. But you can't see it from the tractor seat so wouldn't know if there was a sudden rise in temps (working tractor hard).
But I DO see value in it with respect to having data as to what your engine is doing under various loads.