With respect to tooljunkie, one will be cooler, the blower fan removes heat from the coolant to heat the car interior...so returning coolant will be cooler..
Only if flow is restricted. A full flowing heater core passes through so quick that it has no time to cool much. A few degrees, im talking a discernable difference.
I thought the opposite for a long time, until i did a before/after test.
One warm/one hot- plugged core. Guaranfrigginteed...
Too many years dealing with cars that had poor heat.