i have both the gmc 14050# gvw, w4500 tilt cab with 12' enclosed service body. same as the Isuzu.
the gmc carries much more and is a 350 gas engine. but the mileage was down to 6 in the city.
the 2008 diesel sprinter is a 9990# gvw. the mileage is 13 in our city. and 19 on the freeway. holds a lot less inventory.
truthfully the gmc has a much better turning radius, but the sprinter is a much more comfortable ride.
the gmc was purchased to carry all the toys before i had my own shop/ warehouse. now i don't need all the space that truck provides.
if you're into drain cleaning, and plumbing service and repair, you either need a large truck or you're running to the supply house.
or the sprinter and swapping out the drums with the sectional machines will save a ton of room. stock only the common stuff and leave the rest at the shop/ storage yard, garage.