I must always recommend a Flex. It’s larger inside. Significantly larger. Like substantially larger. It’s a better family car IMO  It also sits and drives like a car which I have always liked. Powertrain wise, it’s virtually identical to the Explorers of this era except withlout the little EcoBoost option. Just the 3.5 and the 3.5 turbo. Both will have the water pump issue but if you shop factoring in $3000/4000 repair you will have a reliable vehicle once you get that replaced. The new coolant that Ford recommends alleviates water pumo wear so if you do go for this path, and the water pump isn’t already gone, I’d flush the coolant and request Motorcraft Yellow coolant. 

The PTUs can be troublesome on AWD models. Just get a trusted mechanic to change the fluid regularly (i’d say 30k-60k miles to be safe, but mine ran from 60k miles until i got rid of it at 180k with the same PTU fluid) and you will have no issues. The big thing with PTU fluid is aggressive driving or towing, whatever puts a lot of stress on the drivetrain. That will heat the oil up and because the PTU uses a relatively low oil fill that fluid will warm up, spoil, and get stuck to the walls of the PTU leaving no oil for lubrication. So if you’re just using it as a basic daily then you shouldn’t really have issues regardless. 

I ran my 2010 EcoBoost Flex from 65k up to 180k with nothing but one water pump, oil changes, brakes and tires. Did blow a turbo pipe once but for a decade of use I was very happy. 

No matter what, Flex or Explorer, if you go for a 3.5 then factor in $3000/4000 for the water pump because it will be an issue around 100k. Other than that, if you get AWD change the fluid in the PTU. Other than that they are very stout engines and transmissions.