For prusa i3 2020 alu frame, whats your suggestion?
@william_foster did exactly what I was going to do. Link to Toms video.
seriously , toms videos are amazing. Should basically watch them all back to back and then try printing.
Having tried both definitely direct.
To answer the rest of the question 3mm seems to be being phased out and 1.75 seems more efficient so I would go 1.75, and I prefer a geared extruder.