Sound quality is only a small part of what makes the movie audio interesting. The sound mixes have been remastered and messed about with considerably since 1997, and it really depends on who did it and how it was done as to what quality is achieved. I've never really compared the quality of the digital media releases.
What's more interesting (at least to me) is the different mixes that existed back in 1977. There used to be a fantastic website called 'The Starkillers Guide' which detailed the differences in the mixes from back in 1977. There were 3 different mixes created: the 6-track mix for the 70mm print, the Dolby stereo mix for the 35mm print and the mono mix for the 35mm print. They were created in this order and as a result the mono mix contains some different sound effects & lines/takes compared to the stereo version. The mono mix was always considered the definitive version and was, supposedly, the one Lucas was most happy with.
A good example of the differences is the 'Close the blast doors' line which only existed in the mono print up until 1997.
