I found our Kelty backpack carrier to be too bulky in tight spaces and crowds to use at WDW. We never took it there, but used it in an amusement park situation that was less crowded and more spacious, where we determined that it wouldn't work for WDW. I always felt as if I were banging into people and things with the big frame carrier in a situation other than hiking, etc....
We also used a combination of stroller and soft child carrier (I've used various slings, a tie wrap, and a BabyTrekker which is similar to the Ergo) at WDW. We like having the stroller in which to put our stuff when the child isn't using it, plus the soft carriers are much more compact to tote with you. Our kids are a bit farther apart in age, but for a while we would bring 2 umbrella strollers or rent a double in the parks. 2 umbrella strollers isn't bad because if you don't need one you can either just leave it parked somewhere or fold it and carry it in the other stroller's basket or on top of the canopy (though one of ours is a Maclaren so its a bit more stable and has the storage room to handle it).
Also, there isn't necessarily room to cart the backpack on rides in all cases but neither is there a place you could "park" it.
Those Kelty packs are nice and comfortable, but I really wouldn't use one at WDW. My younger ones mostly walked or used the soft carrier, then they generally preferred a stroller after the age of 3yo or so.