Do you mind higher engine rpms in the 3000-4000 range on the grades? That's really the question you need to determine for yourself...
The 5.0 is a great engine.. It just produces it's peak power in the higher rpm ranges, so you can't be afraid to rev it when needed..
The #500 difference in "dry" weight figures isn't really going to make any difference IMO.. I tow a #5000 GVWR TT with the F150 in my sig and it does fine, but I have a different engine and rear gear, so I can pull the grades in 4th gear, going 60 mph and pulling around 2600 "easy" rpms.. :)
If I ever get a new TT, my max GVWR on it will be #7500. Don't care about "dry" weights, because between me and my wife, we will stuff it full of stuff and not look back! :)
Good luck!
Mitch