dk1's answer was great. I just want to add one more thing: I think the most important thing to determine when choosing a release code is features. If there is a feature in 9.2 code you would make use of, then it doesn't matter how stable 8.4.7 is, because it doesn't do what you need it to.

I'm going to list the notable features for each code version that I know of, organize by major and minor release. I'll make this a community wiki so others can add to it as well:

• 8.3 - new NAT
• 8.4(2) - IKEv2, NAT done better, Synergy with 8.6.1 for ASAx
• 8.6.1 - Synergy with 8.4.2 for ASA
• 9.0 - SSL VPN Client for Win8 support, NAT between IPv4 and IPv6

(personal note: 8.3 changed the NAT, but you may as well consider it the rough draft and 8.4.2 the final. I would highly, highly recommend not using 8.3 for any reason)