Morgan rose has a beautiful flow to it, whereas alexandra hasn't that much, but, alexandra is a prettier name, i have a friend called alexandra, and she has many nicknames because her name is too long; e.g she has alex, alexxi and alexa,
so if you shortened alexandra when she gets older, like alexa rose, it has a pretty nice flow to it, and its a beautiful name, on its own, and combined with rose.
but, why should you shorten an already pretty name?
i still think alexandra rose, as it has many possibilities.
good luck with the new baby ;)