The only way to know for sure would be to physically check the numbers on the negatives that were used, but that would mean potentially damaging it, and it’s hard to imagine that any Back To The Future fan would believe the value of this piece of trivia is greater than ensuring the quality of the film.
So what we have here doesn’t totally conclude the mystery, but instead simply adds a new layer to it.
What Bob Gale is saying here is that because the reshot material wasn’t given a particularly high grade, it’s possible that Harry Keramidas and Robert Zemeckis opted to use the footage with Eric Stoltz, as it may have been of better quality.