The I-5 Sprinter you loved was very lovable, with the exception of the rust, turbo resonator cracks, and "Black Death" around the injectors. It was also the only factory tall/Eurostyle game in town. All MB V-6 engines that followed for the Sprinter became worse nightmares as far as reliability and maintenance costs. BlueTEC finally added a whole new slew of problems with the DEF complicated systems and sensors. Clean burning diesels are way too expensive and finicky to maintain, especially if it is being used as your home.
I owned and traveled in an '08 170" tall Sprinter for six years, and spent a year deciding on it's replacement after it was totaled since the Transit and Promaster were still just promises back then.
Went with the longest tallest Transit 3.5 Ecoboost, and have 25,000 miles on it now since January '15.
No regrets. Many times this Winter people stopped to ask about the relative merits of the various Euro style vans now on the market.
I asked them how much time they had to talk, and tailored my response to their schedule.
I could compare every detail for hours, but if they need the short answer........3.5 Transit. Done.
And reliability plus parts and technician availability is a HUGE reason.
Also keep in mind...the North American variant of the Fiat Ducato AKA Ram Promaster is not the same animal as the rest of the world gets. Here we are given a Chrysler mini van engine and transmission!