The difference used to be small but as of Mint 12 the desktops in the main editions of the two distros became very different. Ubuntu uses Unity, Mint uses either MATE or Cinnamon - they're all very different from each other.
Mint does some of the common post-install tasks for you with better out-of-the-box support for hardware and multimedia formats (nothing you can't install manually in Ubuntu though), and has a different selection of default applications. The core system is pretty much the same, the main difference being the window manager (compiz >< muffin / metacity) and the shell (unity >< cinnamon / mate-panel).
Thank you for this thread. That’s all I can say. You most definitely have made this forum into something special. You clearly know what you are doing, you’ve covered so many bases. Thanks!