The Madeleine PM can definitely be used as a shoulder bag -- I love mine. But I must say you might want to look at that Tivoli! I have an eye on it too, plus that would give you an LV mono bag which I think is a must! You have good ones picked out!
I'd say the tivoli, because of it's versatility ... love the way it can be handheld or worn on the shoulder. But if you like the mirage as well and don't mind having another speedy ... then go for that one since it's LE. You might not be able to get one later ... the tivoli - you can always get at a later time.