Buffydores,
If you are really wanting some gas mileage out of either of the transmissions you are thinking about, you can always add a "GearVendors overdrive unit". This would give you the mileage you want at the lower RPM's and the high speeds going down the road.
https://www.gearvendors.com/4x4gm4s.html
Just a thought, but all idea's come at a price.