The Z77X-UD5H will be a better board for overclocking, it's part of the ultra durable series too!
Think of it this way:
Power phases are like highway lanes, the less you have, the more chance cars can get jammed in them. The cars can get jammed like traffic, because there aren't enough lanes for those cars to get to. In this case, car=clean power to the CPU, the more lanes you have the more cars you can carry, resulting in more clean power to the CPU!