Harold Kestenholz
As the water in a residential heater does not exceed 160F and should be lowered to 120 outlet temperatures per many codes, it is better that heated water expands into the hot water line rather than sending a half gallon of warm water from the bottom of the tank into the cold line, so placing the expansion tank in the hot water line has some advantage.
However, the Amtrol instructions show it placed in the cold water line in most of the drawings at:
http://www.amtrol.com/products/pressrelease/files/MC4080.pdf
