...I traded in a 2007 sprinter and the body was rusting badly but the underside was holding up well...
That's been my observation as well. Lots of Sprinter haters here complaining about how bad they rust solely on the outer body panel appearance. I've never seen a Sprinter rusted significantly underneath where it matters to safety and crash protection. I find the opposite with many domestics. The body may look decent, but there are sometimes major sections of the underbody rusted away completely and suspension stampings with major rust. Replacing suspension and brake components due to rust is common in my area.
As for Transit, my impression is that underbody panel fit and paint application is no better than E-Series. I would expect the same rust performance as E-Series Vans in your area. Dipping bodies in primer is nothing new. Ford's claims about the color coat "durability" have nothing to do with rust performance. I'm not knocking the domestics' paint processes. They are as good or better IMO as Europe's or Asia's. But, good design, panel fit, welding, and sealing of all the assembled bits is essential to keeping rust from starting in seams. Transit has not excelled in that area.
I predict Transit may rust around the body side moldings since they are attached with trough-body clips just like Sprinter. Over time, expansion and contraction will wear the paint off. Salt will run down behind the moldings and start the rusting process at the clips or at the edges of the molding where it rubs through. Body moldings attached with high strength tape do much better on rust performance.
Door edge rust is one area Transit may fare better than old Sprinters. In general, domestics seem to have that controlled. But, my last Silverado was an exception to that, showing signs of rust on the inside of the door bottoms after only 5 years in salt.