I can't stress this enough - the 3d of the trailers they played were pretty painful to watch, including ones for Shrek and Toy Story, but with Avatar, there aren't that many scenes where its obvious that it isn't just normal life. For the most part it slips your mind that it's a 3d movie because it just flows.
As for IMAX vs whatever the other is - I saw this movie both at Rave which doesn't have IMAX, and at the Easton IMAX and really the only difference is that the IMAX glasses are more comfortable. On screen, it all looked the same. Otherwise, it boils down to crowded ass uncomfortable Easton theater which serves beer, or very comfortable, somewhat less crowded Rave. I think Marcus already switched their IMAX to Sherlock Holmes.