Assuming I have a DC-DC step down/up converter rated at 100-120W. For the input of the converter(accepts up to 32V), would it be advisable to select a 120W power supply of 12V 10A, or 24V 5A?
Would 24V 5A be better due to the smaller current it outputs, and thus producing lesser heat, or is heat completely based on the wattage?
The higher the power rating, the less stress on the circuit.(within reason!) the heat is dependent on the power going through the circuit so a 120W board will be at the end of its rating.
The larger power rating would be preferred.
That does not make much sense. You seem to be talking about two power supplies. Which one is it?
You would not use one power supply to power another.
Sorry allow me to rephrase. The DC-DC buck/boost converter takes in an input of up 5 to 32v DC, and allows you to step down/up the voltage(output). The converter supports up to 120W of output power. So if given a choice to select 12v 10A(120W), and 24v 5A(120W) as the converter's power supply, which one will make a better option or will it be the same, since both are 120W.
I also understand that a leeway should be made(for example choosing 140W) for safety reasons and/or due to the power supply not performing to 100% efficiency. But it's a hypothetical question for now.
Use the 24V 5A. It will minimize the current between the ACDC converter and the buck/boost.