The 970 chipset is an identical piece of silicon to the 990X and FX .
Some are graded higher and become 990's and some become 970's
The 970 chipset in the Asrock extreme 4 should be incapable of SLI a or crossfire , but it can do both because Asrock configure it as a 990X .
So there is no performance difference between 970 and 990 , and only those people who will use two graphics cards should pay for a 990 chipset board .
In this instance where your choices are a 970 board , or a 970 board configured as a 990X you need to decide if you might want to add a second graphics card . It opens an upgrade pathway , but one that is usually poor value since by the time you want to upgrade graphics you can often buy a single newer generation card that would be better than an crossfire set up .
Of the two boards the ASUS M5A97 R2.0 is higher quality and will OC marginally higher . The asrock opens crossfire as an option , but has [ in my experience ] a mildly flakey BIOS , but can SLI/crossfire .
For one card I'd buy the ASUS