The biggest difference between the two engines is described with your quote above. That is the 3.7.I do think the hills around here seem to clobber it when running around with the cruise control on, too. It'll slow down 4-5 mph and then downshift all the way to fourth after a very short time in fifth. You can practically watch the fuel gauge go down at that point.
The 3.5 will not downshift and will hold the speed it's set at. I've only seen my 3.5 downshift towing my trailer loaded with 3-4K lbs in the trailer.
As for fuel economy, when the ecoboost is boosting I don't think it's efficient. I'm 17 mpg light highway. But loaded it's 13 or so. City is 15.
But I like the fact it doesn't have to downshift. Which while people think the turbo's are added complexity, the transmission on a 3.7 van is going to fail a lot sooner than a 3.5 trans that stays in lock up most of the time.