Am not too familiar with those Onboard Raid functions, but as far you want to make a Raid 1 that is mirroring, you need to use motherboard Raid funtion in order to set the array and then get from Asus driver to load it in installation. Then mirroring function will be there.
Also a good idea will be main usage of Raid configuration and what else support.
Am most familiar with IBM ServeRaid, but those are add in cards!!
Just make sure to make some test before enter any important data.