I thought the whole purpose of the new cadetship was to put the entire cost (including flying training) on HECS? If this is the case then it means very little outlaid to start off with- instantly making it more attractive than the REX deal.
Once finishing the QF cadet course you're off to a regional which means you're going to earn as much as a REX F/O. The difference is that after a couple of years (assuming you keep your standards up and nose clean) you're going to end up with a gig in Qantas as a S/O which will earn you far more than a REX captain earns. The pay from there goes up significantly.
By all means apply for both but you'd be insane to pick the REX cadetship over QF!