The J - cradle (left) puts your kayak on its side on your roof, while the flat cradle (right) keeps your kayak resting on its hull. The advantage of the J-Cradle is that with the kayak on its side, you have a lot more space on your roof for other boats or bikes or car top boxes. J-cradles usually are less expensive and many will mount directly to factory cross bars. So they are an easy option. They are also simple to install, and generally in the purchase phase are a bit easier to figure out.
Flat cradles usually require a base rack, you need to make a couple of other decisions - do I want rollers, or something like that on the back?
Both can be purchased for a wide range of prices and a wide range of vehicles. I feel like I see many more J cradles than flat cradles. But there is a problem.
J cradles work great on cars. They don't work so well on SUV's or light trucks, particularly if you are under six feet tall, and/or don't have that much upper body strength. Flat cradles can be difficult to make work for people who own multiple kayak and have small cars, because the recommended cross bar size simply isn't wide enough.
Despite this I see many J cradles on SUV's. Here is the problem. To get your kayak into the cradle you have to lift it above the height of the roof of the car, and above the height of the rack system, and then above the bottom of the cradle itself. On my Yaris it isn't that bad. I have to lift it my 50 pound Delta to just above shoulder height, but put this cradle style on an SUV and you are lifting your kayak well above your head. It isn't as bad with two people, but it is still a chore.
The solution is to use flat cradles on your tall vehicles. Then you never have to actually lift the entire kayak. Lift the bow and put it in the rear cradle - or lay it on the rear cross bar. Then Lift the stern and slide it forward. So much easier than lifting the weight of the entire boat above your head. You can use flat cradles on your small car as well, but you may have to use longer cross bars than is recommended, and then watch your head getting in and out of the car.
I used flat cradles when I had a Toyota forerunner, but when I switched to a small Toyota car I switched to J cradles.
I am not saying that if you have J cradles on the roof of your ford expedition that it's wrong. I just think it is more work than it needs to be. And why would anyone want to work harder than they have to?