I loved Avitar. Don't get me wrong. We say it in 3D at the theater and then went to IMAX.
The movie truly makes you believe in the world of Pandora and it's people, and the visual creativity i the world made me want to travel there! Enough so that we went the second time. It was an experience that everyone should see once - in a theater.
I don't think IMAX improved the experience, however - although it was MUCH louder and several n our group of 10 complained that it was too loud. IMAX put the movie too "in your face" and ,made it hard to catch the nuances around the screen.\
But I would say you have to see it once - where ever you choose to see it.