2005 outback combined city/hwy expected gas mileage (4 cylinder auto): 25mpg
2004 (no data on 2005) Xterra combined city/hwy expected gas mileage: 18mpg
I'd say a 39% difference is significant. And the faster yo go the bigger the difference gets. On a 4 you hwy trip your milage is probably more like 50% better in the Sub.
2005 Outback minimum ground clearance: 8.4 inches
2005 Xterra minimum ground clearance: 8.3 inches
Add the much more comfortable ride, better snow and ice handling, more cargo space, it seems like a much better choice, unless you are spending most of you time off road.