318 or 360

The smaller engine can get great mileage IF it is in a light car. If the 318 is in a heavy truck with big tires, it will be overworked and not get much better mileage than a 360 that isn't working as hard to move the mass.
The longer stroke of the 360 generally means that it will have a lower torque peak, all else being equal to a 318. Less throttle opening to do the same job, though that narrow throttle opening is feeding 42 more cubes...