Ford E-450 has the highest GVWR and allows the longest wheelbase stretch. The recent GM 4500 comes very close.
But you asked about GM 3500... Lower ratings, as with E-350, may affect your decision. I want to offer a consideration based on the little that I know about GM chassis vs. Ford - The Ford Motorhome chassis seems to always include their top transmission offering for the model year in question. In other words, when they offered TorqShift, that's what you got. It seemed to me that some of the GM models, even with 8.1 engine, got a Hydramatic-Something and not an Allison. There's nothing wrong with a GM Hydramatic. Of all the brands we've owned, we had less transmission problems with GM than Ford, even the legendary Chrysler "727." But if you want a transmission with all the tricky goodies, look carefully to be sure the chassis you select has those features.
My understanding may be dated, since it now seems GM is using a Six-Speed Hydramatic that looks like it has Allison-style controls.
Our Ford has the 4R100 that preceded the Allison-like TorqShift. It operates about like the plain Hydramatic would. Has "Overdrive Off" and NOT the "Tow-Haul" with modified shift strategy, engine braking and the like. Works fine, just not as much fun as if it had all the tricks.
If it would fit your capacity requirements, I'd say don't shrink from a 3500.