There seem to be two parallel subjects running concurrently within this same thread: the issue of resolution as limited by film plane positioning accuracy and lens quality between 35 & 120 formats; and the issue of tonality differences as related to negative size verses enlargement factor.
As I read the original subject title, this thread has to do specifically with tonality differences between the two formats, not resolution differences. But on the subject of film plane accuracy, I would agree that, in general, 135 format positions the film more accurately than 120 format; a broad-brush statement, to be sure, as there's always exceptions to be found for both arguments. For instance, I would expect a Rolleiflex may have the edge over a plastic disposable 35mm camera despite the advantage that exists intrinsic to the 135 format film itself, simply because the Rollei has been engineered to a greater degree than the disposable camera.
As for tonality and its relationship to print size, the enlargement factor is of major importance. With all else being equal, a contact print of a 35mm negative compared to a 24mmx36mm crop from a 120 format contact print should have the same tonality; the two should be indistinguishable. And certainly a full-frame 120 negative enlarged 'X' amount should have better tonality than a 135 format negative enlarged the same 'X' amount (with all else being equal.)
Where it becomes less certain in my mind is when comparing, say, a contact print of a 135 negative against an enlargement of a 120 negative. Certainly more detail will be visible in the 120 enlargement merely because of the print size, and the 120 negative should intrinsically contain more tonal information than the 135 negative; but will the diminutive 35mm contact print have better tonality (a different quality than resolution) than the 120 enlargement (depending on the enlargement factor obviously) simply because the opaque silver grain clusters that make up its image are closer together?
