Definitely heat. It is easily transferred through metals and as the metal heats up it will also give off heat. Cold is slower than heat. When it comes through metal, It tends to "frost" the metal and not give off near the same amount of cooling that it would if it were heat going through the pipe
I am too tired and lazy to give a scientic definition, besides, not my forte... but I would say Heat. Something can heat up (not boil) just heat to be white hot almost instantly. Flame can be produced instantly. Cold creeps. (unless we are taking liquid nitrogen & then I don't know my theory might be blown!)