Upgrade from Debian Wheezy or install Debian Jessie in a clean way?
Back up all the important data first.
My situation is that I have a personal computer I can work on. I also have a hard disk to back up the data.
I chose to install Debian Jessie in a clean way. I read the release notes. Many differences exist between Debian Wheezy and Jessie.
If I upgrade from Wheezy, I need to handle issues such as old config files.
Sometime later when I encounter problems, I will not be able to tell easily if it is a problem of upgrading or a problem of installing Debian Jessie.
Upgrading can cause more issues in my opinion.
It is best to install in a clean way.
Format the / partition and other system partitions as ext4. Install Debian Jessie.