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

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

There are a few factors to consider those two:

3mm

• More rigid - easier to print with flexible plastics. I couldn't make my 1.75mm 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

1.75mm

• More popular, easier to buy
• Needs smaller extruder than 3mm
• Easier to use with bowden tube
• More precise when printing

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