Unless you have a very expensive hardware raid card, the raid functions offered by the bios are known as fakeraid because they are really software raid pretending to be hardware. It is in fact, a bog standard AHCI sata controller with special software built into the bios and a special driver for Windows to perform raid in software. As such, there is no performance advantage over pure software raid, which is what you get when you set it up with the Ubuntu installer or disk utility. The only reason to use the fake raid is if you must dual boot with Windows, since Windows will not recognize Linux software raid. Otherwise, it is not as well supported as plain software raid, and so should be avoided.