Are you pressing the pedal to the floor as often as possible? Driving around with three obese friends? Up hill, into the wind?What is the MPG going to be for these engines?
Gas engines are most efficient (thermodynamically, i.e. chemical energy to mechanical energy) near wide open throttle (WOT); the more closed the throttle, the more throttle losses and the higher the specific fuel consumption (SFC=fuel burned per unit power produced). A smaller engine operated with a more open throttle will generally be more efficient at low-load conditions (i.e. highway cruising). At high RPMs, these throttle losses are near-zero, but the friction losses are high, so the sweet spot is usually cruising at ~2300 RPM in top gear with the throttle mostly open. This, combined with the lower weight of the 2.3L vs. the 2.7L will probably work out to a ~10% longer highway range with the 2.3L.
Diesels don't have a throttle, which is why they are such efficient highway cruisers (combined with diesel just having more hydrogen molecules per gallon of fuel than gas AND diesels having high compression ratios). They are most efficient sitting at low-load conditions.