We take my 07 Yukon XL 11 hours each way for our annual Spring Break Hog Hunt, with a hitch & haul. I get the best mileage of the year on that trip. I've never used a cargo carrier, but I can't imagine the mileage being as good. I'll also give a nod to the earlier poster who mentioned wind noise - I'd expect some.
The hitch & haul handles a few rubbermaid containers that hold bulky (but not too heavy) stuff, like clothes, waders, etc. Now, we drive straight down & unload, so we're not trying get stuff in & out of the back regularly. Still, I'd have an even harder time trying to climb & twist up to the luggage rack to get stuff out of a cargo carrier up there.
We have always had 3 of us on the trip, and we've packed the truck to the gills every time with gun cases, coolers, food, & junk. This fall we're going as a party of 4, so we're going to rent an 8' enclosed trailer.
If you do decide to go with a a cargo carrier, I'd look first at a sleek Thule-type bullet carrier, rather than one of those X-Cargo cubes, or the rubber-sack-looking things I see on minivans that are screaming north on I-75 out of Florida.
Good Luck,
FC