System76 comes with Ubuntu. You just need to add KDE or kubuntu-desktop and
switch the session to KDE or Kubuntu when you login. It is far easier than
re-installing. Installing Mint 13 would be a hassle and you get no
advantage. You get a couple of Mint applications and Mint theme and lose
the advantage of being able to upgrade. Mint has no upgrade tool. You must
do it yourself or re-install when you want a new version.