I'll offer a couple of my thoughts, since I'm in the process of looking to make the same kind of switch that you are.
Currently in an F150 FX4 and looking for a van type option with full stand-up space that can haul cargo and serve as a more comfortable camping space than the short bed of my truck with a topper. I've looked at the Sprinters and Transit vans as well as older ambulances.
I am leaning away from the Sprinter for a few reasons. First and foremost, the 4x4 system on the Sprinter is limited to something like 35/65 torque split and there's no locker. Granted I've only used the locker on my truck a handful of times, but when I needed it, I was glad I had it. I drive in snow a lot. I'm not sure how much of a difference 35/65 vs. 50/50 would make, but I'm used to a true 4wd system and I drive in awful conditions frequently. Second, I'd prefer not to go with the diesel for reasons already mentioned. Third is the price, both of the purchase and of maintenance. I think the Ford would be the more economical choice.
That said, the Sprinter has some features that I really like. One is the option of the crew van set up, which has two rows of seats, just like my truck. You cannot get this option in the Ford. Also, the Sprinter can accommodate larger tires than the Transit. The Quigley prototype van is rocking 31" tires I believe but there is pretty much no room for chains and they said they had to do some mild trimming to fit them. I think the biggest you could go would be 29" or maybe 30" without any trimming on the Transit. I think the stock Sprinter tires are 29" maybe, and I'm thinking you could go to 31" or 32" with a lift, but I haven't done my research on that for the Sprinter.
As for the Transit, I like it for the EcoBoost engine, the cheaper price, the (supposedly) better handling, the option for dual sliding cargo doors, Sync3, and ease/cost of maintenance. I don't like the lack of a second row of seats as an option or the tiny wheel wells.
Check out QuadVan in addition to Quigley. They are also an official upfitter so you can have your van drop-shipped to them to do the conversion just like you could to Quigley. The difference is that QuadVan gives you the option of a 4" lift and they will also install a locking diff if you want and they will install an electronic transfer case with shift-on-the-fly capability (unlike Quigley). This is something that appeals to me, although I understand how Quigley's manual transfer case is appealing to others.
Maybe some of that helps, maybe none of it does, but as I'm looking into making the same switch, those are some issues that have come up for me in my search.
