Interesting read. I guess it all depends on how strong a filter you need to get to the right exposure length (2-4 minutes in his case). If he's exposing such that a 6-stop gives him 2-4 minutes then a 10-stop would be a nightmare as it'd give 32-64 minute exposures. Conversely, my examples were around 1-2 minutes with a 10-stop and would thus have been 4-8 seconds with a 6-stop which wouldn't have been long enough.
It's down to what you shoot really. I can believe that, if he's on about landscapes around golden hour, there'll be much less light available so a 6-stopper might be better. My shots were all during the day in good light so I needed more stops.