Well, this is easy. If you have v5, I can't really recommend buying v6. I have found no appreciable useful difference between v5 and v6. Perhaps it is better in expanding disk performance, which I don't use. Actually stopped using expanding disk because fixed size was way faster. Then I moved to virtual on bootcamp.
Don't get me wrong. Parallels is a great product. If you are buying it for the first time, there's no reason not to get v6, but I'm not sure I'd pay more than $10 for an upgrade and that just to get to ride the upgrade/patch train.