AFAIK Alitalia is the only airline that gives you full flown miles for any paid class of service no matter which ST airline you're flying. And since you are in MEL the fact that they give (full!) miles for QF sectors should make your choice even easier!
Don't listen to anyone who tells you they're going under...They won't be failing anytime soon....they're expanding and I'm starting to see more people aboard their planes... ^