The 70mm picture quality could potentially be quite nice, though with an aspect ratio of 2.2:1, some image would be missing from the sides.
The soundtrack is quite the most desirable aspect of it, of course. I know there is at least one privately owned 70mm print of the first movie out there, but nobody seems to know who actually owns it. Most likely this person would be unwilling to loan it out for scanning. I’ve never heard anything about whether there are any existing copies of the other two.
Re 70mm, I have seen two 70mm Star Wars prints, only one was in projectable shape and both were far too faded to red to be useful for restoration of the image, it is extremely likely that they all are far too faded now.
Plus 70mm is cropped at the sides, so I don’t think it will be a lot of use for the visual restoration.
Any 70 prints would be Eastman likely and virtually all from that vintage are typically in tatters or so faded they are unprojectable. Additionally the mag tracks can go if not properly stored.
Imagewise, the only benefits would be the finer grain and what if any visual changes were induced through the blowup–albeit those you could tell through heavy fading. The big draw will be the Dolby 70 baby boom in the six track mag stripe. If it could be found, transferred and turned it would become a most powerful ally. 😉
But of course all this would be much easier if we could access the source and do a straight transfer, much like some 70mm mixes that got direct transfers to LD and DVD. Like the earlier Criterion 2001, any magnetic we might be able to work with would likely have some distortion or damage after all these years.
The IB will be the best image overall and the most filmic/organic feeling. The Eastman/LPP will be what you’d have seen in 1977 and what the movie should look like from the processes of the day on general release.