It's really the power the engine needs to produce to go down the road. You can produce that power with fuel intake at either a slower RPM or a bit faster one.
The difference comes when you are using the power in a gear range. Assuming you reach your top gear at highway speeds, if you have shifted into high well before you got to highway speed, you will likely stay in high when a hill comes in front of you. If you live in flat land, go with the lower ratio, but if you want to not have your tranny shifting up and down, go with the higher one.
