I have two kids (7yo - 10 trips WDW and 3yo - 3 trips WDW) and have flown there 9 times, driven once. I've never gate checked a car seat -- too much hassle dragging the thing around. I've used the CARES straps when old/large enough or held in my arms during take-off/landing and otherwise used a carry pouch.
As for the backpacks, I don't think any particular brand/style matters as much as the basics like a good quality zipper, strong seams, compartments that match the size and shape needed for what you need to carry with you and one that fits you and hits you at the right spot so as not to cause a backache.
I've used at least one of my son's Disney-themed backpacks from a prior preschool/school year/summer camp every trip for one of us to use as a carry on -- then we use later for our daily trips into the parks. We literally wore one of them out after the fourth or fifth WDW trip and gave it a little goodbye ceremony before we tossed it in the trash.
I have also purchased a "hiking" type backpack that is larger and more rugged to use as a piece of luggage. If I don't overstuff it, it fits carry-on requirements. But honestly, it is too cumbersome to easily get out from under the seat during the flight. At the end of the day, I prefer the easy to find (who can miss the backpack with the big Mickey face on it) and easy to access child's school-size backpack.