dfk41
David, partly addressed in my reply to Jake. It isn’t the 3mm; it is the geometry which is entirely different and as you point out is much more important.
98mm are inherently large and expensive, and even their most fervent devotees seem to think better for filter, but no improvement on 80mm for espresso.
75mm (and some others) are oddball sizes often confined to a single grinder – or burr–vendor.
80mm has multiple grinder choices (see current list above) with more on the way, and seemingly a healthy choice of burrs and vendors (and likely more on the way here too).
If Niche wasn’t so excellent this would have been an easy decision. Fingers crossed, Niche Duo 80 ;-)