Hmm...3mm and 1.75mm should be the same in terms of pressure, since the same diameter that increases the melt volume also increases the retract volume. I suspect the real difference comes in terms of viscosity. In the smaller bore, the melted filament will have a much harder time flowing around the air bubble pulled in by retraction. It'll be much easier in a 3mm bore for the filament to flow around the bubble and ooze.

Is there anyone who has tried both and can compare? I've got an awful lot of 3mm filament to make the jump.