I was never opposed to the idea of a double flap for the jumbo and maxi since I own a reissue 227, which has the double flap, and is like my everyday bag. The extra compartments (in front of the main compartment, the space behind the interior flap, the zip compartment, 2 card size slots, and lipstick holder slot) make organizing my things easier than with one big compartment with only a zip compartment and an open slot. This is assuming the new jumbo and maxi will share the same interior characteristics of the 227 and 228, which remains to be seen.
As for keeping its shape with the double flap, hmmm, I've seen some reissues that are slouchy and winged. If you're constantly opening and closing the flap, it will eventually wing slightly due to Sir Isaac Newton's Law of Gravity. On the other hand, seasons after seasons, Chanel makes changes to material, giving varieties to those who prefer softer leather and a slouchy look available. Keep i mind that leather softens over time due to use, because leather is a natural organ, and, as such, stretches. I am more convinced that it is the underlying thickness of the cardboard used in making these bags more structured. The sides of both the jumbo and maxi will always be more squashed, as this is the look; otherwise, the bag will be very boxy and bulky to my liking. Other factors, such as humidity, also play a role as well, breaking down the cardboard and softenng the leather . Proper storage is important to maintain the bag's shape. There is a section on proper storage of bags, please read it as it contains many valuable information.
The reason the medium/large is more structured is due to its size, putting less weight and support of the cardboard; thus, reducing the likelihood of wings to develop under the flap. Look-wise, I prefer the medium/large, but in terms of practicality, it barely fits anything to suit my everyday needs. This is where I prefer the reissue 226, much bigger to fit my essentials; yet small enough for evenings out.
As for weight, I don't find the reissue 227 heavy by all means, unless I overstuff it, and that's painful for my poor shoulders. I guess I am looking forward to the new double flap icon bags, Chanel changed the classics to icon bags btw. I am certainly not thrilled about the price though.