You will most likely want to get the ".tgz" version. The netboot image is usually used only in recovery circumstances, and the .zip file is used when upgrading from ONTAP 7.x.
Regardless, make sure you read, understand, and follow the instructions in the Upgrade Guide! If you have AutoSupport enabled, MyAutoSupport has an upgrade advisor which tells you about any configuration gotchas, etc. prior to performing the upgrade from old version to new version.
If this post resolved your issue, please help others by selecting ACCEPT AS SOLUTION or adding a KUDO.