The MPOWER is among the most expensive Z77 boards and from a straight OC'ing perspective you're not going to get your money's worth out of it. The Extreme4 is a killer in the "bang for your buck" category, but honestly I like the ASUS P8Z77-V LK (my board). It is well-priced for what you're getting which is solid OC'ing ability, the right number of ports and features, and the best LAN/audio chipset.
Take your pic between the ASUS and the Extreme4 and you can't go wrong.