I am trying to understand the difference between qcache option and mris_fwhm and which is appropriate for a cortical thickness analysis.  I processed my  files with qcache and have lh.thickness.fsaverage.fwhm15.gii (converted) files.  I used an afni tool SurfFWHM to estimate the smoothness of a subject at when looking at the fwhm0 image it iwas 5.5 and for 10, 15 and 20mm it was approximately 9.3-9.9 smoothness level. 

I also used mris_fwhm --hemi lh --s fsaverage --smooth-only --i lh.thickness.fsaverage.mgz --fwhm 15 --cortex --o test_15.gii  and when using SurfFWHM on the smae subject the smoothness was estimated at 11.25.