Both are good chassis. I'll echo the previous responses and say the coach whose floorplan and amenities best suits your needs and preferences is the one to choose. Chassis, really, is a secondary consideration to finding a coach you can live in.
That said, assuming similar service records, mileage, and condition, either chassis is good. There are many (many) people here with both Workhorse and Ford based MHs. Either will do a fine job or propelling your MH down the road, and either can be serviced by any competent RV shop.
2002 National Dolphin LX 6356
Workhorse W-22 chassis
Don't believe everything you think.