So, I'm building my second printer based on the AM8 plans on Thingiverse. I'm thinking of buying either these 0.9deg 46Ncm motors (here) or these 1.8deg 59Ncm motors (here). My current printer uses 1.8deg motors, and I was wondering if I could increase print quality with 0.9deg motors on all axes? To do so, I have to sacrifice 13Ncm of holding torque, and I'm not sure what holding torque value I should not fall under.

Any recommendations?

stick with 1.8 for non delta type machines

Overly simplified, using 0.9 does give you more theoretical resolution... but at the cost of half your max speed (twice as many steps/mm and you start to run into atmega limits, unless you also upgrade to 32bit controller)

Ah okay, thank you for the information. 1.8 it is then!