I'd be more inclined to go with 5.8 with the latest patch. 5.8.1 was a fork to support the new appliance models.
I know that 5.8 introduced the generally more stable new AD connector code vs the old Centrify that was under the covers of the earlier versions.
If you are subject to audit or care strongly about internal security, 5.8 Patch 4 also added support for TLS 1.2. The other 5.8 patches fixed a whole bunch of application vulnerabilities.
Some more browser incompatibility issues have been fixed. I've found 5.8 to be much better than earlier 5.x versions in that regard.
All this and more are covered in the release notes:
http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_secure_access_control_system/5-8/release/notes/acs_58_rn.html