NIS has all features 360 has, only the backup and the tune-up tools (cleaners and the defrag feature) is missing from it. If you like to control everything in your app. then NIS is again better for you as with it you have a much more controlable program (it was designed for power users vs. 360 was designed for the average user who didn’t like to change anything…).
As you are currently using Norton 360 2.5, there are two choices ahead for you:
1. Renew the Norton 360 2.5 subscription, then update it to the latest version - Norton 360 3.0 for free of cost via the Upgrade Center.
2. Get a new Norton 360 3.0 upgrade and install it over your current version when the subscription expires.
I am not sure about the pricing for full package that you get from local stores, but if you feel that it is much cheaper when compared to the renewal, you can go ahead for it. Make sure that your CD package is not tampered when you purchase it from local stores. If you need any more clarifications regarding the prices for Subscription Renewal/Upgrade, don't hestitate to contact Symantec Customer Service. They will assist you further and provide the required information.