While the boundaries are soft in the second exposure, I can't see a meaningful difference in graininess or acutance between the two.

To be honest, I think you might be beyond "direct" visual comparisons at this point - which is to say they are very close. There might still be small differences in image structure between the Concentrate and XTOL, and they might influence the subjective perception of granularity (ie graininess) and acutance in photographs, but to evaluate this more objectively you're into micro densitometer territory (ie RMS granularity).

Just my two cents.