People tend to start with 3mm filament, 5 mm nozzles, 0.4 mm layer height.
-A bigger size means more tolerance of hardware shortcomings (like a not perfectly flat bed, no glass) and mis-manipulation.
- You can get a finer resolution look (0.2 mm) with a trick on skeinforge and other slicers that halves the layer height just for the perimeter.
- As of lately experienced people are printing very fine layer heights even with large nozzles.
- Until recently, 3mm filament was clearly more affordable than 1.75mm filament. Now you are starting to see similar pricing per weight.
- You need more force to push 3 mm filament into a hot end, so you need a heavier, geared stepper. If this must fit on a moving carriage, this means more inertia, less speed.
1.75 mm filament can be efficiently pushed with a Nema 14 motor (see Adrian's extruder)
One advantage I can see with 1.75 mm filament is that you need more steps per mm of extruded filament, allowing your extruder to be more accurate in the amount of filament extruded while also maintaining its ability to push filament quickly. If you want to make really small features with 3 mm filament you will have to push the filament a very small amount, which is hard to do accurately when taking into account filament springiness. On the other hand, 3mm filament is thicker and thus less prone to buckling under pressure.
When I bought my machine parts I was asked the same question 3mm or 1.75mm and 0.34 or 0.5mm nozzle. I opted for 3mm and for the 0.34mm nozzle.
My machine is now 95% ready and I was wondering does the fact that I ordered parts for a 3mm extruder means I can print ONLY print with 3mm or can I print using both filaments sizes?