Ive never been a fan of MSI motherboards. Asrock has good quality boards and loaded with features. The quality and the features you get are well worth the money, awesome bang for your buck.
Iv always used Asus motherboards and probably always will so i can't really comment on the Bios or much of the specialized features etc.
But when overclocking i would recommend a higher end board anyways.
Differences in PCIe slots and splitting lanes aside, the ASRock has a few extra features. It has an eSATA port ( which I love, ) two extra USB 3.0 ports, two extra SATA III jacks, and a USB 3.0 header in a friendlier position by the RAM slots. The MSI has a sixth analog audio output and coax digital audio, neither of which I care about.
You tell me if those differences are worth the 15 in your eyes.
just get the extreme4. more better power phases (fixed crappy power delivery problem found in older asrock boards), will support SLi which helps crossfire users, and generally more complete in terms of a feature set.
i personally like the msi bios and there is a OS that can be good for web browsing and that sort included. good for when you dont have windows disk