Whats the difference between real d 3d and imax 3d?

The fact that it is in 3D is irrelevant to the question. Imax theaters have much bigger screens (it is pretty impressive), and the movie in this case will be converted so that it will look amazing in the Imax theater. Personally I wish more film-makers spent more time perfecting Imax rather than 3D (which is what christopher nolan is doing for the dark knight rises). It is always awesome, whereas 3D is ocassionally awesome and usually just uncomfortable. Titanic sounds like it could be really cool in Imax, if you have the extra money go for it. One thing though, be prepared for the huge screen. Meaning if there is a way to reserve specifics seats (the imax theater near me has that), reserve seats as far back as possible of the theater and in the middle of the row. If you can't, then try to get there early so you don't have to sit in the front. Being in the front in Imax is terrible, you can't see the whole screen and you do not get the full effect.

The difference-other than the huge size of the Imax screen is about $5 -7 more per ticket for Imax

IMAX 3D = huge screens, good image quality, most expensive compared to the others, huge linear glasses, very good three dimensional pop out effect. RealD = ciruclar classes, 3D is not as good (you have to adjust and maintain your head in the right position for the 3D to take effect), focuses on digital format and still relatively new, doesn't pop out as much, I prefer IMAX over RealD anyday, but it's quite pricey. You should go for IMAX if you can afford it and want the best possible 3D experience. For the price, you should check with the theater you're going to, but RealD usually costs about $9-12, while IMAX is a couple dollars more than that. Hope this helps!

IMAX is a way bigger screen... like a lot bigger.. Id see it on the IMAX... is saw it on regular 3D and it was a really good movie.

iMax 3D is basically just 3D, but with a much bigger screen.