It depends more on how much time you want to spend on post processing and how detailed the part is. Teller layers means more time spent filling and smoothing.

You can print at .2 layer height with a .6 noz and benefit from a thicker line width, giving you stronger parts but the overall time to print will still be similar compared to a .4 noz.

Check out the youtube channel Frankly Built. He’s got a ton of cosplay How-Tos, especially helmets. He has some good suggestions about print speed, printer types, supports, and post processing, which will all impact what settings you use.

https://www.youtube.com/c/franklybuilt

If you can afford it, sometimes printing the bulk of the helmet with a big 0.6mm or 0.8mm nozzle, and then print the high detail components with a resin printer.

But also, depending on your setup, a high quality, super fast enclosed XY Bambu Labs printer at 0.4mm will print faster than an old Ender 3 bed slinger 0.6mm printer, and have less post processing.