the saleya does not lose it's structure, at least not at the base. when i'm in a crowded place, my MM gets bumped a lot even if it's not even half full.
Both the Saleya MM and GM will lose their shape eventually, especially if you carry heavy items in it.
As for the Sac plat, I think it would maintain its shape better, but I have no personal experience with it.
As for the Sac plat, I think it would maintain its shape better, but I have no personal experience with it.