IMAX 3D screens are known to be big. They are known to be bigger as compared to the screens that are being used for Digital 3D. Those that are being played in IMAX 3D may still be in analog form while those that are digital, from the name itself, are already in digital form. The type of polarization that is being used by these two is also different from each other.
Digital 3D makes use of circular polarization while the IMAX 3D will make use of linear polarization. Take note that you would need to wear 3D glasses for both in order to fully maximize and enjoy the movies that you will watch.