They both have distinct advantages and disadvantages. If you are doing mild offroading I would definitely go for the Transit. Disclaimer: MILD offroading is all that 99% of us will ever do in a van. They're simply too big/tall/expensive/etc. for heavy off-roading. The two biggest reasons are fuel mileage (It will be a solid 5 mpg more with the Transit, maybe more) and ON-road ride. A Transit drives like a minivan. It is totally smooth and comfortable on the road, I mean sedan-like, not truck like. The V-10 with Ujoint will be a beast capable of getting you anywhere but is not going to ride as well as a Transit on the road, or be as smooth and quiet. The Ecoboost has the same power as the V10 practically, and is far more than you need unless you are towing heavy things.
I could go a lot deeper but I'll just say this. Many won't like it and that's fine. My reputation here is well known as an E-series van fan who bought a Transit hesitantly (as a work vehicle) and more or less fell in love. Here it is: Time marches on.
The E-series has a lot of plusses that the Transit (or Sprinter or Promaster) may never live up to. It is essentially bulletproof. It is body on frame and all of it's issues have been worked out of it since it's been the same for 25 years. We're in year number 4 for the North American Transit and upgrades and improvements are being made left and right.
Sadly for us all the E-series is disappearing. Your investment in an E-series will not have the staying power that an investment in a Transit will have. Do you care? That is the question. The Transit is the new E-series. Ford isn't going back at this point. They are selling Transits like nothing else to folks like us and fleets, service industry, shuttles, etc. It's going to get harder to sell an E-series and after a few weeks driving a Transit you will definitely see the datedness of an E-series. The layout, controls, mirrors, headroom, climate control, suspension, transmission, efficiency, and virtually everything else are worlds better.
I've rambled on plenty. I look at E-series every day and will probably buy another for something or other but it will be for a specific purpose when I do. Just make sure of your purpose.
There is one more person who frequents this forum who is a fan of both and has made the leap to Transit. He is far more technically savvy than me and his input might help you out. Hopefully he will chime in. Where are you Michael Gabriel (MGmetalworks)?