Which size chassis is both of them on:
The Ford based motorhome - E350 or E450?
The Chevy based motorhome - 3500 or 4500?
The Ford chassis (whether E350 or E450) of that year will have their superb TorqueShift computer controlled 5-speed (6-speeds internal) transmission with it's automatic down-shifting Tow/Haul mode for effortlessly safe mountain driving.
If the Ford is also the optional heavy duty ton and a half E450 chassis, it's a no-brainer to choose the 2006 if the rig's tank capacities, condition, equipment types, construction quality, and floorplan are up to what you want.
Be sure and also check gas tank capacities between the two motorhomes. Ford's will probably be 55 gallons whether an E350 or E450. I don't know what tank capacity will be with a 2004 Chevy chassis. I do know we travel with friends who have a Class C motorhome on a chassis with only a 36 gallon fuel tank ... and it's a REAL PAIN for them to have to stop and gas up what seems like "all the time" when traveling with the typical Class C gas mileage of only 9-11 miles per gallon.
Remember ... when buying a motorhome (whether new or used) ... many details often turn out to be regretted, or appreciated, only after owning and driving it quite some time after you've bought it.
2005 E450 Itasca 24V Class C