Let us know what you decided!
I just joined the forum and am late to this discussion but want to add my 2 cents for others facing the same decision who may read this thread.
Part of the decision important to me was ongoing cost of ownership:
An oil change on a Promaster runs about $100 for full synthetic and you can even go to Jiffy Lube or Valvoline. My Transit owner friends pay $350 - 800 unless you can DIY, and those Jiffy-type places can't do Sprinters.
And there are way fewer certified Mercedes Sprinter mechanics in the U.S., compared to Dodge. A friend of mine once had to have his Sprinter towed 1200 miles. Yikes. Probably not typical, but still.
Promaster transmissions have a reputation for earlier failure, but the lower purchase price makes replacing it kind of a wash. One guy told me it cost him $5k. At 60,000 miles!
The diesel Sprinters do have a reputation for longer engine life. And a bit better fuel economy, though diesel fuel costs more. I get about 15.5 mpg on my PM.
I like the Sprinter's higher roof, though you feel more sway in a cross wind. The narrower width requires a lengthwise bed orientation. I was able to orient my fixed bed width-wise.
Lastly, the Sprinter walls are more curved, making cabinetry etc a little more tricky. But if you're planning to hire an experienced builder, they would have templates for either vehicle.