Here's my personal opinion.
If you want stability and don't need any of the features of 6.0, the 5.5 code is going to be a better choice. It is mature and they are still working on bug fixes.
6.0 adds some really useful new features - which are too numerous to mention. I want the new NC split tunneling option, the Citrix/RDP enhancements, etc. However, I'm waiting (impatiently) on 6.0 since my house isn't burning down.