6.5 is current Instant software version. 8.3 has just been released weeks ago, and the 8.x versions will be where the new features will be developed. Over time, you should consider moving to 8.x.
For now, if you have a stable 6.x deployment, I would upgrade to the latest version in 6.x, unless you need features that are only in 8.3 as described in the release notes.