Condition, floor plan, and price are more important than the chassis manufacturer at a given MH age.
I have been very satisfied with my older V10 (plugs are fine). Power and 4 spd tranny have been fine. Yes, the increased power and transmission speeds would be nice on the 2006 and newer chassis. No way, would I spend the $$$ to trade my older one for a newer one.
Lots of comments about the Allison tranny, but I have rarely seen posts about Ford tranny issues. All I've ever done with mine is put in Mobil 1 tranny fluid soon after I bought the MH at 32,000 miles.