Here is a comparison from Pickuptrucks.com that compares the 3.5L Ecoboost against the 5.0L on the dyno. Now before anyone takes the numbers to heart...understand how subjective dyno numbers are on any given day...and don't use this one comparison as an end all be all of how they perform. (They explain this in the summaries due to different dynos etc.) It is just an interesting read.

How We Dyno Tested Ford's 3.5-liter EcoBoost V-6 and 5.0-liter V-8 Engines - PickupTrucks.com News

Personally I will get the ecoboost for the low end torque while towing. Hose toy you have to go drive them both...and I don't mean on a straight highway for about 5 minutes. The 5.0L sounds great and does have really good power...the ecoboost is deceptively quiet and when you look down you can't believe how fast that speedometer is rolling to the right. Honestly on the street torque is where it's at. The 5.0L will pull your car easily just as the ecoboost will...the question is are you in the future going to pull anything bigger? Besides the low end torque, the ecoboost also gives you more towing capacity.

You could get away with 3.31's towing that weight which will get you better unloaded economy. (Towing up steep grades will most likely need a downshift at this gear ratio...is that a bad thing?)