hairy_hen said:

The 35mm stereo mix was most likely the first to be completed, since it contains the least number of sound effects compared to other versions, and may have required the most testing in order to ensure its technical compatibility. However, it and the 70mm six-track would have been created at virtually the same time, since the actual original mix was not the stereo version itself, but a four-track master from which both the stereo and 70mm mixes were derived.

The stereo would have been created by downmixing the four-track into a matrixed LtRt, with peak limiting applied to fit it into the reduced headroom of the optical format, and the panning width reduced somewhat to avoid excessive crosstalk between channels when upmixed again during playback. The 70mm version contained the four discrete channels but with unrestricted dynamic range, and with the addition of bass content in the other two channels generated by a subharmonic synthesizer in order to bolster the impact of explosions and spaceship flybys. A very small number of additional sound effects (four, by my count) were added to the 70mm that are not present in the 35mm stereo, but for all intents and purposes they are virtually the same mix, simply optimized for different presentation formats. Listening to either will serve you well, depending what kind of sound system you have.

....what he said. :)