I picked up an Apex in the shop and it was a nice bat. It's not got a low swell point like the Luna, or one that is higher up like the Flame and Icon. The Apex is in the middle of those bats. It's a general all round bat, whereas the Luna is more for those who like very low middles, and the Icon and Flame are aimed at those who like higher middles. Plus the Apex has a nice arch as well. Depends on your game and how it feels when you pick it up. But that's not always possible if you don't have a good cricket stockist near you.
But this GM chart might help;
Apex has replaced the hero so would guess a driving bat
I thought the Luna replaced the Hero?
Since the Hero had a very low swell, and the Luna is now the only GM bat with that. Plus Graham Smith used the hero, now he's using the Luna; so I suspect it was the Luna that replaced the Hero.