To me, it really depends on what you are towing and how comfortable you are with towing anything when nearing the vehicles MAX capacities..
Just towed my little #5000 TT with my new F150 and all I can say is WOW... :)
I towed the same trailer with a 97 F150 with 5.4 for over 11 years and thought it was a great tow!
Well, this Ecoboost F150 handled it awesome! Going over the same passes, it never went below 4th gear or above 3000 rpms.. I had to watch my speed, as it wanted to tow at 65 in 6th and if I didn't watch, or put it in cruise, I'd find myself at 70.. Going up the grades, 60 was easy and I have yet to put the pedal to the floor..
Anyway, use the right tool for the job and you'll be happy.. I think I'd be comfortable towing up to around #8000 with my new F150.. Anything over that and I'd want a bigger truck..
JMHO on it all.. :)
Mitch