I am replacing my motherboard and just wanted to get an idea of which would be best to replace it with. Kepp it $300 or less I mostly game and listen to music aside from obvious web surfing. My build is as follows
With your sound card, pick one of those boards with the ports you need. I never recommend spending too much on one board; I'd rather get a spare board (two of the same for quick recovery; any board can go bad). I worked in an IBM printed wire board factory for 9 years; there isn't that much difference in quality between a $300 board and one that goes for $150.
They may have more ports, a different power phase (for overclocking), and one has better onboard sound, but you have a separate sound card, so that's not an advantage. If you're well off, then you can get one of your first choice boards; nothing wrong with either one.
On the Asrock you can run two GPU's at X16 speed versus X8 on the Asus plus you can have 4 GPU's at X8. Not that there is going to be a noticeable performance difference between running X16 or X8 with multi GPU setups.