Flip a coin! Quite a number of people on the various forums say they will NOT upgrade to 7 because 6 is working too well for them. If your needs are basic burning etc.I would suggest upgrade the 6 version. I upgraded several 6 versions over the top of older ones and had no problems. Some others had problems that way. It is a crapshoot. Try uninstalling first using the clean tool from Nero. Nothing like a fresh canvas.
On the other side of the coin, there are a couple of versions of 7 that are stable and cause no problems. Some versions are VERY buggy. If you surf this site and another at www.afterdawn.com
you will find a zillion pros and cons to each version. In the end it is up to you to decide
As far as installing Nero 7, run the clean tool, download the current version from the website and use your serial number that's in your CD when it comes to the page where it shows a serial number on the screen. Delete that one and insert your own. I have the 184.108.40.206 installed and have had two small glitches.....nothing to sour me too much. "IF" you don't like 7, just uninstall it and go back to 6 and re-install it. Anyway.....search a little while and read some posts but with all that, I still think you should toss a coin. You have a 50 /50 chance of getting a good version