Gas or diesel class c

If you are specifically talking about an E350 or E450 based C, the advantages of a diesel (if you can find one) are pretty minimal. Both the 7.3 and the 6.0 used in the E-series are detuned significantly from their pickup truck counterparts and in my opinion actually offer less performance than the V10. The exception to this would be at high elevations. I back this up from both having owned a V10 powered C, and also the fire department I work for recently switched from 7.3 diesel ambulances to V10 gas. Although the rigs are virtually identical, the V10 absolutely destroys the 7.3 in performance and at least equals the one shop queen we had with a 6.0. Now of course fuel mileage is a different story, in the ambulance application the V10 gets about 6 mpg vs. 8-9 for the 7.3.