Ortleibs and similar Rixen and Kaul quick release fitting are the best way of fixing a bag. Nothing else comes close.
My Carradice bag is tough and practical with opening in the correct direction for access, a map holder on top that really speeds up navigation and QR buckle closure than can be overloaded. I have one internal zipped security pocket and 2 organiser pockets and the whole bag is really a rigid correx box covered in fabric.
Size should fit the bar style and size. I dont like really big ones, they encourage you to carry too much in the front. Ortleib is about as big as you should go.