Obviously, you've got to like the floor plan and overall livability of whatever coach you choose. And, the mechanical condition and how each of them has been maintained is critical.
Assuming either floor plan works for you and both have been properly maintained and are in satisfactory mechanical condition, I'd vote for the 42' Dynasty with tag axle. If you are going to be traveling a lot, the tag axle coach will provide a level of ride, handling and weight carrying ability that will exceed that of the 40' coach. I'm a proponent of buying high end used when you can due to build quality of most higher end rigs. The Dynasty was high in the Monaco pecking order and, IMO, in a class above the Tiffin Bus.
Tiffin folks will tell you that Tiffin offers outstanding customer support even for coaches well out of warranty.
Either of them would not be a bad choice.
2019 Grand Design Solitude 384GK 5th wheel. Glen Allen, VA