Hi there,
I know alot about both of those airlines. REX is really good, pays okish and conditions are good. QF Link has a few problems...pay is lower to start with and the moral around the Sydney base is pretty low at the moment too. At QF link you will have the opportunity to work with another crew member (sometimes) which some crew like, however at REX your a solo flyer so you have to be very confident in your ability as a flight attendant.
However saying that, I think you should give them both a go...Over all I think there pretty much the same...with just a few minor differences!!
Good luck
Anne X