Back in its day, the Workhorse chassis seemed to be leading with the innovations, and Ford was playing catch-up. The Workhorse chassis also came with a $3,000-$6,000 premium in price, depending upon your dealer, so there were fewer of them ordered. Most people didn't know or care that there was a difference.
Both the Ford V10 and GM 8.1L are good engines, and I wouldn't worry about either one. In my mind, and the minds of many who ordered their motorhomes with the Workhorse chassis, is the fact that that WH chassis has an Allison transmission attached to it.
I can't speak to parts availability issues as I haven't experienced that yet. On the other hand, our rig hasn't had any chassis-specific issues, either. 8.1L engine items and Allison transmission parts should be available for a good long time.
I think most people have more issues with the coach part of the motorhome than the chassis, anyway. Winnebago/Itasca is a good brand with LOTS of technical documentation (plumbing diagrams, electrical diagrams, etc.) on their website, which in my mind, is a HUGE PLUS!!!!
To be honest, if the floorplan works for you, THAT is more important than the chassis it is on. Obviously, general condition and any maintenance records are important as well.
Good Luck,
~Rick
2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (27-Angel since 2008), 1 girl (22), 2 boys (23 & 20).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.