What's the biggest trailer you've towed with your Expedition?
My 18' (box) travel trailer which weighs ~6000lbs. But it's tongue heavy at ~930lbs, which means I've maxed out the receiver rating (white sticker on the receiver) even though I could theoretically tow a trailer weighing 50% more.
I'm going to say something biased and apocryphal here: the Ford is the better tow vehicle. The Ford 3.73 rear axle has a weight rating of 4380lbs, while the GM rear axles are only rated for 4200lbs (3.08 is only 4140). Max tongue weight on the GM's is less than 800lbs. And keep in mind that that higher rated rear axle is under a lighter weight aluminum body.
For towing duty, the 5.3 V8 just can't compete with the 3.5L Ecoboost (go look at the power and torque curves). So performance wise, you'll be wanting the 6.2L V8. But that's a heavy engine and lowers the payload down to 1580lbs vs my 1670lbs*.
Sure, there's lots of people who will argue that a solid rear axle is better than an independent rear axle. Sure, in theory. But you're buying a real truck with real engineering compromises going into the design of the rear axle. And in the real world, the Ford comes out on top.
*More if I'd gone without that awesome sunroof. But I live in Seattle and it's already depressingly dark outside most of the time and the interior is black and...