None of them need a support. A lot depends on your frame size. I have a Nelson Longflap that will graze the rear tire on a 23" (58cm) frame if loaded with items that make it sag. Putting a plastic stiffener in the bottom corrects this. So does using fenders, or a support like a Bagman. Not an issue with 24" frames. The Lowsaddle is for smaller frame bikes, like less than 22".
The long flap is darn handy. It stays tucked back most of its life but on the day you want to carry two or four more things, you just extend the flap and use the second set of straps and you're still in business.
Search on "Bagman QR" to hear different opinions on supports.