You’d have to drive both but I can tell you having had an F150 with the 3.5 ecoboost (of the Wonder), it is a very responsive and capable engine with a lot of towing capacity, and with more hp AND torque than the MB Sprinter diesel (of the Unity). Given that, in this case the diesel would be a tough sell for me. And I generally prefer diesels. Also the Ford transit has an AWD option which would be a very nice feature on a coach. Not sure about the Sprinter but its not listed as an option.
The Unity offers three options I would prefer (Alcoa wheels, propane genny, and jacks) that aren’t avail on the Wonder, but in this case I think the power of the Ford 3.5 gas and optional AWD would still carry the day. That said, I just went through the build and price process on the LTV website and the Wonder is $32k more for the exact same build with no options. Interesting.
My neighbors have a Unity (now has around 60k miles on it) and we’ve traveled with them all over the state (coast, mountains, and desert), lots of hills, grades, etc, both towing a similar load (Polaris Ranger on a small utility trailer) and the Unity was right there with me all the way. And they love it - no complaints.
The gas/diesel question has been debated extensively but in this little niche its still a very valid question.