So here's my experience:

IMAX 3D: lower frame/second (48), 1.78:1 (the real IMAX ratio is 1.44:1), 4k screen resolution (Avatar's original screen resolution is 2k)

Digital 3D: pretty high frame/second (72), 2.35:1, 2k screen resolution

I believe both versions are pretty good, but D3D has amazingly sharp image quality (but smaller screen.) I have enjoyed both versions, and of course Avatar is the best film I have ever seen