Resolution wise the lower the number the better, but that increases time. so a 0.40mm will make a sharper print then a 0.50mm (but the 0.50mm will complete the print the print faster). Speed is also determined by your printer / extruder.

Should I sacrifice build volume for resolution


No reason to, the nozzle should be changeable, I bought a Makerfarm 12" (Kit) for under $800 and it came with a build volume of 11.5"x 12"x 12.5" (roughly 300x300x300mm) and a 0.40mm and I could go smaller if i wanted to.. although I hear that the smaller nozzle can have problems with clogging.