Now this I would love to see the answers from the guys here, I've heard both ways with backing arguements for each.
For hot water...
1- lower pressure of the coil after a control valve may cause conditions where fluid quality to goes down. ie vapour may form in the coil (noise, cavitation, oxygen)
2- the valve and actuator of a valve in the supply line will be much hotter than the return line (hot actuator = bad)
3- supply line means more chance of destructive cavitation at higher vapour pressures.
For cold water...
1- the supply line may well be below the local dew-point which would mean the valve and actuator may form condensate (water in actuator's moving parts = bad)
dew point of 22°C @ 50% is 11°C (return may be like 12 °C, and flow may be like 7°C)
...bunch of good reasons there
1 + 1 = 3 ( *** for very large values of 1)
...everybody wants a box of chocolates and long stemmed rose