I have the 0.4 Hexagon for 3mm filament. I chose the hexagon because I plan to print with nylon at some point. I wanted the ability to print PLA, too, so that ruled out the Magma.

When I told Colin I was leaning towards 1.75mm, he indicated that he thought the 3mm might be a better choice for me. I did not press him on this, so I can't tell you why he said that, or if I even understood him completely. The 3mm is working for me, and I have no complaints.

It does seem, however, that there are more sources and color choices for 1.75mm filament than for 3mm filament. If there is a downside to the 3mm, that might be it.

The additional resolution available with the smaller diameter 1.75mm filament makes sense, and is something I had not thought about before buying. OTOH, in my experimentation with ESTEP settings and the equivalent values in slic3r configurations, I am not able to see noticeable differences in print behavior across a fairly wide range (+/- 5% or more) of extrusion rates.

I, too, am anxious to read what others have to say on this subject.

Jim