There are a few factors to consider those two:
3 mm
- More rigid - easier to print with flexible plastics. I couldn't make my 1.75 mm printer print with NinjaFlex using standard Bowden extruder.
- Can extrude faster
- Quality is less affected by changes in diameter - it is a concern mostly when using cheap plastics
1.75 mm
- More popular, easier to buy
- Needs smaller extruder than 3 mm
- Easier to use with Bowden tube
- More precise when printing
