Definitely see it in real IMAX 3D (no comment on the smaller, LieMAXes). The RealD was a little more fluid, but it's not worth losing 25% of the screen, and the IMAX version definitely felt more immersive. The IMAX versions have the full 1.78:1 cut, while the rest have 2.35:1. I haven't been able to confirm what cut the LieMAXes show, but from what my friend said, it sounds like it has the 2.35. It's possible it varies from LieMAX to LieMAX as well.
To the GP, personally, I thought it surpassed Star Wars, considering the relatively limited time he had. It definitely fleshed out the love arc a lot more (although that would have been ultimately extra creepy if Lucas had as well). The story is complete. Cameron doesn't need to make two more if he doesn't want to. I'm hoping there won't be sequels, since I believe sequels usually ruin things (there are exceptions).