Just about every film that appears in imax is not shot on imax. It’s optimized and blown up so they can charge you the premium imax price. That’s why when you go to see a film in imax you see small black bars at the top and bottom of the screen. It’s become something of a ruse to squeeze more dollars out of folks.
A lot of bigger tent poles (Batman, Star Wars, Black Panther, etc.) will shoot select scenes on imax but not the whole film.
Avengers Infinity War was totally shot for imax. That’s a film where you almost have to see it on imax to get the full experience. The last transformers was shot entirely on imax 3D cameras.
RPX is straight and better on cost. The one I’ve seen had comparable sound. Screen wasn’t quite as big but it filled the entire screen.
Now if you want a real premium experience where they aren’t articfically blowing up the picture the Dolby Cinemas at AMC are supposed to be top of the line sound and picture with those new laser projectors.