First, (if I may ask) -- how many passengers, cargo, and the goal (transport only, camping, etc.) ???
I'd love to drive (or even ride in) a Sprinter diesel for the experience
I agree -- it will give you much better FE (but...)
With the features you mentioned, it may be costly to rent (also figuring a prediction of the added cost of Diesel fuel).
It all depends on the one you need and can get.
Chevy Options:
The Chevy 1/2-ton (LS 1500) comes in a 5.3L rated 12/16 MPG, but at only 8-pax.
Then the 3/4-ton (LS 2500) and 1-ton (LS 3500) bumps the engine up to a 6.0L (and 12-15 pax). Likely low teens (EPA not required to rate).
Avoid AWD.
Ford options:
The E-150 hauls 8 with a 4.6L V-8. The 12-15 pax model (E-350 SD) is now a "Super Duty" model with the base 5.4L V-8 or the V-10. The 5.4 carried a 4000-lb payload in a moving truck quite well during a move this past year. 9 MPG, but plenty of power. They make a diesel version, but I havent seen any for rent.
Comfort:
The Chevy/GMC may offer more comfort -- the Ford can get pretty basic (and the 1965 technology of the twin I-Beam front suspension can get keep you busy at the wheel. I drove a fleet of '93 non-turbo Diesel Ambulance conversions for many 1000's of miles). I drove a Chevy Van recently and it was much more comfortable the Ford's I've rented.
Just be glad they don't make the Dodge 15-pass van anymore. Yikes...
RH77