Should I format and upgrade to mavericks or upgrade to mavericks and format? I am planning to upgrade on a Unibody Macbook (2008) to Mavericks. Mountain Lion is already installed and I felt it to be a bit sluggish when I upgraded. I also would like to start anew and clear some files that have been on the machine for a few years. So I am wondering:


Would you advise formatting on Mountain Lion, adding files and apps back then upgrade to Mavericks?
Would you advise formatting on Mountain Lion upgrading to Mavericks and adding files and apps back to the Macbook?
Would you advise upgrading to Mavericks then formatting and then format then add files back?
You could pick this route:


Back-up everything you need
Format your disk (this will format/clean Mountain Lion and all files)
Install a clean version of Mavericks
Put back the files you still need


Why this way? 
Easy, you won't have the old Mountain Lion files on your file system. This results in a clean new environment to start with.
The other methods you described will result in Mavericks on top of Mountain Lion.