I've read reviews of the ab-doer and the biggest complaint was that thingy on the back that is supposed to 'massage' you. Well, most people felt that it was giving them a noogie.
Of the sitting type ab workers I would suggest the abswing. You can pick one up at Target or Walgreens these days.
I've never used the Gazelle, but I personally don't like Tony Little.
I do have an eliptical and I LOVE it!