MPower Max has more phases in its power design than the MPower ( 20 vs 16 phases ), plus 2*8pin cpu power header instead of 1, so it'll give out better results if you want to do some extreme overclocking. Beside that main different, Max version also has more usb3 ports ( using NEC's usb3.0 controller ) and a TPM header. That's all. Other features are identical between the 2 boards.
http://www.msi.com/product/mb/Z87-MPOWER-MAX.html
http://www.msi.com/product/mb/Z87-MPOWER.html