The Ford will have more capacity. The Chevy will ride better empty. The 6.0 6-speed will use less fuel however the 4-speed doesn't have that same advantage. The E350 V10 has more much more aftermarket support than the any motor in the Express.
V10 E350s have become very hard to find however, and command a pretty good price premium now. Express vans are much easier to find, and the 6.0 was the standard motor in the 3500 series, so even rentals got that, which helps finding them used.
I would recommend a 2005+ V10 simply because its matched to a better transmission. Same with the 6.0. The 6-speed is far superior to the 4-speed.