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.
I agree with Kevin. Do not upgrade unless you really have to. There are so many horror stories about upgrades both here on the boards and amongst other SSL VPN owners that I'm not doing it until I can see that the upgrade isn't going to break LDAP, AD, Citrix, NC or something else that I've had to fix after an upgrade / downgrade.
Don't do an upgrade unless you absolutely have to.