The MH-505's have better balance and a more natural response, but the Z-560's can blow out windows if you want them to, plus the bass is unreal if you crank it up. We're talking floor vibrations, wall vibrations, windows rattling, the whole nine yards!
If you are a true audiophile, get the MH-505's. For everyone else, the Z-560's will blow you away, and for less than $200 they're a great deal.