3.5 will be plenty to lots of people but lack in towing power, sticking to 5.4.
Some people will always have the opinion that a V-8 will always be better than a V-6, even when the V-6 makes more torque and horsepower.
The V-6 is perceived as being less than. But here is some food-for-thought, there was a point in time when early trucks and pickups didn’t have V-8s. The biggest engines offered were up to six-cylinders. Even when eight-cylinder engines made their entrance into the truck world, the six-cylinder engines remained very popular for a long time.
Old advert from Ford for its "new" six-cylinder powerplant
More food-for-thought? Torque ouptut ratings are in descending order, from greatest to least:
2015 Ford Expedition .............. 3.5 EcoBoost V-6 ........... 370 horsepower,
430 lb-ft of torque
2015 Mercedes-Benz GL 450 .... 4.6 BiTurbo V-8 .............. 362 horsepower, 406 lb-ft of torque
2015 Toyota Sequoia .............. 5.7 i-FORCE V-8 ............. 381 horsepower, 401 lb-ft of torque
2015 Dodge Durango ............... 5.7 HEMI V-8 ................. 360 horsepower, 390 lb-ft of torque
2015 Nissan Armada ................ 5.6 VK56DE V-8 ............. 317 horsepower, 385 lb-ft of torque
2015 Chevrolet Tahoe .............. 5.3 EcoTec3 V-8 ........... 355 horsepower, 383 lb-ft of torque
2014 Ford Expedition ............... 5.4 Triton V-8 ...............
310 horsepower,
365 lb-ft of torque
With 430 lb-ft of torque, the Expedition will accelerate like a rhino on crack, and easily get a heavy multi-axle trailer up to speed. Much faster than the 5.4 Triton V-8 can.