[Piwoslaw]

How to climb hills has already been discussed here:
Hill Climbing
Engine load vs rpm? (Climbing: hold top gear at very high load, or downshift?)
Hills (most efficient climbing technique for diesel / manual transmission?)

You have to see whether climbing the steep hill back, or taking more gradual route, uses less fuel. Not just better MPG, because distance also plays a role.

It might be that one of the routes forces you to go faster, so any savings from more efficient hill climbing is lost to aero drag. Or it might have more stops. If you drive this way regularly, then maybe make notes on the fuel consumption for each of the two routes and which direction (exact distance and MPG). The more numbers you get, the better idea you'll have.

You may even refine the method to take into account time of day and weather, for example one route might be more efficient when you are using A/C, the other would be better when you aren't.