Both are equally good. But since x79 is newer it sports quad channel memory. Four ram slots, better pcie X16 slot options, etc but it us also more costlier than the Z77 board.
The best X79 board is ASUS Rampage IV $430
ASRock X79 extreme 11
Best Z77 boards
Gigabyte G1.Sniper 3 $270
Gigabyte Z77X UD5H $250
ASrock Z77 extreme 6
Actually z77 is newer. But these are aimed at different markets. X79 is for the high end market vs z77 which is for mainstream. Also currently z77 is IB and x79 is SBE. It'll get IBE but by then haswell will be out so still a generation behind.