To Gang's response I might add that estimating the
smoothness of the noise from a single volume (and
via division no less) is itself a very noisy computation.

Estimating the smoothness of the noise from the EPI
time series does it from hundreds of TRs (and times
the number of subjects), so the estimate should be
far more robust. And since both estimates are
attempting to estimate some sort of larger voxel size
based on that smoothness, it seems fair to think that
both methods are trying to estimate the same thing,
particularly from the viewpoint of Monte Carlo
simulations..

- rick