I read and looked at a lot of stuff when I decided I needed to get a new van. I really liked my T1N Sprinter, was skeptical about NCV3, but the new I-4 motor is supposed to be OK.
BUT, diesel isn't the long term value it used to be. Current emissions standards have rendered the diesel in full size vans to be only marginally better than gas for MPG, and the extra maintenance cost (DEF, larger oil amount per change, etc) pretty much makes operating costs the same (assuming current fuel price fluctuations making diesel and regular gas about the same price averaged over the year). And extra equipment means extra things to break. So, I decided that gas was the way to go, for now. Mercedes doesn't have a gasoline engine offered for the Sprinter.
To add to Dave's assessment, I find that my Transit is much less maneuverable than my old T1N, and rides harsher. I have never driven an NCV3 Sprinter. My Transit drives like a huge box truck, instead of car-like as my T1N did; I have to forgo a lot of parking spots because I know that it will be a 3-4 point turn to get into them, where in the Sprinter I just drove right in. The Transit IS a little wider, though.
And if MPG is the primary concern, the Promaster diesel has been reported to be averaging above 25MPG in combined driving. The diesel ProRam only comes with the intriguing computer-shifted manual transmission, it drives like an automatic but it's a real manual transmission; the computer activates the clutch and shifts it instead of you. Kind of like push button start; the computer decides when it's time to let off on the starter. Some people really hate it.
Another thing in favor of the Ford; they have regular discounts for various reasons, and sell off the lot for 10-25% less than sticker price. The only time you'd pay MSRP is if you ordered one. My van's sticker was almost $35k and I paid about $26k (discounts for 2015 model year, Labor Day, new buyer, etc). Sprinters are hardly ever "on sale" and you pay sticker most of the time. That's one of the main reasons I bought the Ford, it's the best deal for the money in my opinion.