What is the basic visual difference between 15:70 IMAX and Digital IMAX?
Two visual differences:
a) 15:70 IMAX has a taller screen, whereas Digital IMAX has a low, wide screen like traditional movie dimensions. This means that if anything filmed in 15:70 IMAX is shown in a Digital IMAX theater, the top and bottom of the image will be cropped off.
b) 15:70 IMAX has much higher resolution (the 70 stands for 70MM film in the projector), allowing for much greater sharpness and detail. Digital IMAX has resolution superior to a normal projector, but inferior to that of 15:70 IMAX.
This site explains it well:
One of the pictures from that site gives a good depiction of how the top and bottom of the image gets chopped off:
The size of the screen might be less of an issue, because with Digital IMAX everyone is seated closer, making the smaller screen look larger. But the top and bottom will still be chopped off of any image filmed in actual 15:70, and there will be less sharpness and detail to the picture when it's showing 15:70 footage.
Of course, all this only applies if part of the movie has been specifically filmed in 15:70 IMAX. If nothing in the movie has been filmed in 15:70 to begin with, then a Digital IMAX theater will work just as well as a 15:70 IMAX theater.
So far, I think only Batman, Mission Impossible, and now Star Trek have had actual 15:70 footage in the movie.