Katahdin
Platinum Member
Fuel efficiency edge goes to the Yanmar engine, its direct injection and has a larger bore and shorter stroke. With those two factors you're going to use less fuel which will probably negates the slightly smaller tank. You haven't said how long you plan to keep the tractor, but my money would be on the piston rings and bearings lasting longer in the Yanmar before a rebuild is needed. Of course, I'm slightly biased towards that brand.
Enjoy whichever tractor you choose!