Wow...so much mis-information.
There are more WH because they sold a lot more - it was an option most went with when they were new because it was a much better chassis. At least up to where they got bought out and shut down.
I have a '05 with a 5spd allison, no 'auto-park' but does have the grade-brake which shifts the trans down for you on steep decents.
As mentioned, the allison alone was enough of a difference. The V8 had lower rpm peak torque, and sharper wheel cut, heavier duty frame and suspension as well.
I would agree with the comment to find one that suits and not worry about which chassis, either is fine. I had a V10 in a 99 Southwind and it was fine too. The 8.1 was just a little nicer, ours is ten years old this year and still like new - and not a single issue in all these years. (except the burnt plug wires but that's an easy fix).