Alright!
For fun, and as best I could, I engraved the “shades of gray” image with Grbl 1.1h and Lightburn, using the same material (chipboard), feedrates, and resolution (0.2 mm and considered “best” at the time) used when testing/discussing issues with Marlin 2.0 (bugfix) back in the “…not getting great results” thread back in Feb/2019.
SIde by side, the results WRT “shading” are [to my eye] reasonably similar at feedrates of 20/25/50 mm/s (top to bottom) and resolution of 0.2 mm… Grbl/Lightburn (left), Marlin/ImageToGcode (right)
Of note, the motion of the laser carriage is far smoother with Grbl’s laser mode ($32=1) than the Marlin 2.0 of Feb/2019… without the starts and stops (and overburn) at each power change across the shaded blocks of the image, especially at 50 mm/s.
With Grbl 1.1h and Lightburn…
and with Marlin 2.0 (Feb2019) and ImageToGcode…
I’m really not sure the current “state” of Marlin’s laser capabilities – judging by Roger’s results it seems way better – and for non-laser functionality and fancier features, Marlin may be the better, more versatile, choice… but, for laser engraving (which is mostly what I mess with these days) and basic CNC, Grbl seems to suit my need(s) nicely.
Just my $0.02… and – as always – YMMV! 
– David