NEMA17 0.9deg/400-step motors are easy to source and cheap, though if you want one with dual-shaft so you can do manual adjustment you'll be disappointed.
I don't think there is a general rule about 400 vs 200 step for load handling. Each type has a full range of capability.
One thing that is a bit annoying about 400 step motors is they move at half the speed! A 400 step motor on a 1mm pitch rail moves agonizingly slowly. I have a 400 step motor on my KR26 rail with 2mm pitch, and it takes a while to move the full range. I'm even considering changing it to a 200 step for this reason. I can live with 10um basic step, with 2.5um quarter steps, for most of my work below 10x. I can just piggyback a voice coil motor if I need finer steps. I recently picked up a 100mm SKR20, which has 1mm pitch, and plan to install a 400 step motor to see if I can get it to approach the resolution of the voice coil motor, but over a wider range. I can envision the SKR20/400 mounted on a KR26/200 to give quick movement plus ultra fine steps.
Edited to add: looking over the Lin stepper motor page, it does seem that the 1.8deg motors in general have higher holding torque than the 0.9deg, but both are available over a wide range so I don't think there is a limitation in our application.
Also realized that it actually takes very little holding torque to keep a vertical setup stable due to the action of the drive screw. I am using the smallest motors I can find for my vertical setups, and they work fine without movement. It takes a lot of force to move the unloaded rail at all, and even just attaching a motor without powering it up makes movement almost impossible.
I did see this on the Lin site, for those wishing even more resolution...800 steps, 0.45deg, but only in NEMA23:
https://www.linengineering.com/products ... dard_motor