Ford Flex TV - ecoboost or no?

We have both versions of the 3.5 V6. Ecoboost in the F150, and it has PLENTY of power. The wife's Edge has the non-turbo 3.5, and it runs fine while not towing. Towing a very small ATV trailer (<2000 lbs total), you can sure tell its back there.

Having driven them both at elevations over 10,000 feet, I see no difference from almost sea level (600-700 feet) when not towing. The fuel injection, computer control, and variable cam timing on todays modern cars takes care of the previous AFR altitude problem inherent with carbureted engines. The turbocharging (NOT supercharging, big difference) is just the icing on the cake. If I was buying a Flex, I'd opt for the Ecoboost if you're going to tow often.

I'm not a huge fan of the Ecoboost but for what you're trying to do I would buy the Ecoboost. A supercharged engine just tows so much nicer in mountains.

Well you already have experience with the Ecoboost. If you liked it, buy it in the Flex. Personally, I would not want the naturally aspirated V6, towing or otherwise.