I am contemplating switching over to 1.75mm. Not because the 3mm is harder to get but based off of info I have been able to find online. The most compelling argument I have read is that the 1.75 is extruded with less pressure at the hotend and would print with a higher resolution. These are not very scientific reasons cause one would assume that pressures would be the same and that the resolution would be the same. I do know that the 1.75 would need more filament per mm to extrude the same amount so perhaps the resolution argument has some merit.

I just like to tweak things is basically why I want to switch.