The AP-315s support AOS 6.5.0.0 as well as 8.2 and 8.3, so you have flexibility in that regard.
AOS 8 provides the most value when running with a Mobility Manager (MM). The Mobility Manager can be either a hardware or virtual appliance, and there are HA options for the MM as well, but since you did not mention that component, is it correct that you only have the two 7010 controllers and the APs?
Without the MM, AOS 6.5.4.x is likely your best scenario. You can run the two controllers as either master/local, or as redundant masters. The primary difference will be whether you want active/standby failover with a single IP address shared by both controllers (redundant masters), or would rather do active/active redundancy with APs split across both controllers at once.
Are all 9 AP-315s being deployed at the same site, or are there multiple locations all connected back to the single central data center?