Upgrading server OS and BE on same server

I currently have a server running 2008 R2 and BE 12.5D with a tape drive. I need to upgrade the OS to 2012 R2 and we've just purchased BE 2012. What is the best way to upgrade both platforms? I was going over a couple of scenarios of either upgrading BE and then doing an in-place upgrade of the OS, or doing a clean install and then importing all the media.

Thanks