Which one should I get and why?
Extreme 9: http://www.scorptec.com.au/computer/48140-z77-extreme9
Maximus v Formula: http://www.scorptec.com.au/computer/46842-maximusv-formula
it comes down to what you want in a board (sli, not getto onboard audio, Usb3) stuff like that you cant say one board is better than the other because it comes down to your needs.
So post your needs and then we will go from there because the asus board for example has really good onboard audio while the asrock board has more usb3 ports.
hope it helped!
asrock has:
8 sata3 vs the asus's 6
8 rear usb3.0 vs asus's 6
2 front 3.0 headers, vs asus's 1
2 ethernet ports vs asus's 1 (supports teaming for faster connection speed, in theory)
also comes with a front panel that includes the wi-fi antenna and a pair of usb3 ports, also has less pcie 1x and pci, but has 3 more pcie x16
Depends if you want to OC as well, and how experienced you are with that. The ROG uefi bios is very easy to OC but also very deep if you want it to be. The asrock is harder to use, but in the e end is still usable.