IF the line waits were the same - I'd vote for Astro Orbiter. (I have 2 DS's too, now much older than yours) I like the height, the rocket theme, and they're "retro" cool (ok, Dumbo is retro, too).
BUT - as others have said, the AO line is VERY slow moving and BORING. We ONLY do it when lines are 10 minutes or less (park opening & close). I also agree with PP who said that the Magic Carpets should be thrown into the mix. It's a similar "go around in a circle, make carpet go up and down" type of ride.
As far as queues, Dumbo is hands down the winner, no contest. Your DS6 should like it.
I'm also going to mention that none of these have ever been "must do's" for my family. Even at a young age, my DSs preferred going on haunted mansion, BTMRR, pirates, splash, or SM a 2nd time over AO, Dumber, or Aladdin.