Why would I not want 0.9 degree steppers

I don't think there is a disadvantage. It is just that there is not much of an advantage. In a typical hobby-level CNC, precision outpaces accuracy in almost every case (especially with micro-stepping). So imrpoving precision alone is usually not a huge win. If your precision is 0.04mm and your accuracy is +/- 0.1mm then moving to a system that is 0.02mm precision without improving accuracy doesn't actually matter.

-Jonathon Duerig

 

If they are the same price, there is probably no reason not to go with them. I went with the 'high torque' motors from the part store.

When I was researching how much to microstep, I recall reading that there were sometimes problems if you didn't microstep at all. Maybe that there is a higher chance of vibration and resonance with no microstepping. It seemed like there was a happy medium with some microstepping, but not a huge amount. On my current machine, I use 8x microstepping on a rack and pinion and don't have problems with either torque or vibration. But I cut relatively soft materials (plastic mostly with some plywood). So maximizing torque might be very important if you are working on something that could cut metal.

 

1.8 are better for torque they do have less accuracy but where talking small numbers.
0.9 have less torque but are more accurate at high speed.
honestly i would use 0.9 on 3d printers lasers **** that doesnt have forces fighting them.
and use 1.8 on things with spindles