Arne,
Do like the rest of us and install them and compare for yourself. 6.5 adds some re-branding to contribs.org, secure mail add-ons, https only to server-manager. Some other under the cover things. I found everything I had running under 6.0 ran fine under 6.5 without any changes. One thing that does break in 6.5 is the ability to log all mail. 7.0 changes OS's to Centos 3.4 (RHEL 3) IMO, this is a big change, but a change for the good. Just about everything I have the runs under 6.5 will have to be re-worked for 7.0. 7.0 adds clamav, spamassassin, horde 3.0, some new proxy settings, and a whole bunch of other stuff. Your best bet is to go to the wiki page and see what rpms are being included. You will be pleasantly surprised with the progress. I have had an issue with upgrading from 6.x to 7.0, but I haven't spent enough time to see if it's me or a bug.
GL,
JB