Yup, Debian Testing it is. I used Stretch-testing until Stretch-stable was released. Since then I’m in stable, but plan to upgrade again to testing in a few months (I thinks this is a reasonable stable upgrade pattern).
Anyway, I still think of Debian as a GNU/Linux distro for a non-newbie, so Ubuntu (or Mint, I’ve never tried it) should make you familiar with some stuff before.
Edit: BTW, I’ve found completely by chance the manufacturer of Why! and System76 laptops. It arrived to my RSS reader, I wasn’t even looking for it. Although this is offtopic, I’ll put it here for now because I don’t know yet where to share it. The name is Clevo and seems to be a Taiwanese company: http://www.clevo.com.tw/clevo_pro.asp?lang=en (those N240xx laptops match with my Why! N240JU and System76 Lemur)