From what I understand most of the improvments in Jira 6 are in the UI. Revamped view issues (reducing unneeded heavy page refreases while searching/filtering) and project creation streamlining. There is also some good stuff with sharing and downloading workflows.
oh oh! and jira mobile has been improved a lot (if that is an option for you).
Advantages of upgrading? UI refresh, mobile and sharing workflows.
Check this out: https://www.atlassian.com/software/jira/whats-new
Hope this helps.
In order to get the new features of JIRA 6 and resolve few issues that were on previous version. You can have a look at the following:
Hope that helps !!!
As long as your Greenhopper/Jira Liceneses are valid there won't be an extra cost. after upgrading jira you will need to upgrade Greenhopper through the UPM detailed here:
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