If someone is looking for a NEW gasser, so why worry about it? They only have one choice and it is the Ford chassis, period. If they want to go to a diesel, than they will have more choices. They cannot get the Allison transmission on the NEW or used Ford chassis.
Since you are looking at a used coach, that is already out of warranty, then look for the floor plan that works best for you. Both engines will get you where you want to go and both will get you over the mountains. In some chassis configurations, the WH chassis has a greater towing capacity than the Ford. The Workhorse brake issue has been dealt with and is no longer an issue. The fires in the Ford engines has been dealt with and is also no longer an issue. Parts are available for both chassis and 99% of the items that you will need are available off the shelf at most parts stores. Some like the Allison transmission, but the newer Ford transmission is probably just as good.
Find a floor plan that suits you and forget about the chassis make because you will never wear out either one of them.