Both will do the job. Both get about the same 7-8 MPG. If the 8.1 has the Allison trans that would be a big plus over the ford trans. You don't give the year your looking at, there it can make a difference also. The ford in later years with the 5 and now 6 speed trans is up to pare also.
I have the Ford V10 2002 and it has never had any problems. I agree with Len on the transmission. I did go to synthetic trans fluid as it will handle heat better and heat kills transmissions. I think the Ford has a better cooling system than the Chevy. I am a Chevy person but didnt let the Ford be a deal breaker. Either one depends on previous owner care and service. new models only have the Ford chassic I think. Len is correct on MPG. I average 8 but did get 13 MPG crossing Texas with 40 MPH tail wind LOL
. Mine is a 33 ft PBD HR