I just upgraded from my K4 to the PW2 and have the same question.
If I were to use the "personal archive" from Amazon (Send to Kindle), I would of course choose
mobi (both) if applicable/required or the
old mobi-format. Simply because
Amazon doesn't accept any other eBook-formats.
Currently I'm using Calibre and sideload my books via USB and would usually just keep the input-format and convert them with KindleGen or Calibre, if required.
Same request: Please, point me out, if there is something wrong or a better way.