The one I have is an Imperial, I would prefer another imperial simply so it will match the OTHER imperial one I have, but a Happ or Atari unit will also be acceptable.
The one I have for trade is in good used condition. My camera is currently vacationing in Guam, but its an imperial trackball, it looks like any other imperial trackball. I'll toss in a plain metal mounting plate with it (not black, it is just metal, needs paint, or if you are using an overlay and have extra material then you can cover the plate with your extra overlay material, which is what I usually do).