My opinion is, it all depends on how you drive. If you have a tendency to really mash the gas pedal as you climb a hill, it would be better to use the cruise control. If, on the other hand, you are able to ease up on the gas as you climb the hill, you might actually be giving it less gas on the way up than the cruise control would.

Also, on the way down the hill you need to let off the gas and coast down, maybe even braking, in order to do better than the cruise control.