Hello,
I would recomend 6.01 if you have a production server, 6.5rc1 is a beta.
I did an upgrade (on the same disk) from 5.5 to 5.6 to 6.01 and I have had great luck with 6.01, add in the spam and antivirus filter from
www.sme.swerts-knudsen.com and you are in business. We have about 200 users and everything runs great.
From what I have heard 6.0 is very similar to 6.01, so I wouldn't suspect you to run into a problem, except for the webmail bug (check the bug tracker).
Either way, make sure to do a full backup first.Good luck, and let us know how it goes!