Stepper motor speed (RPM)

I am trying to build a lab vial roller like the following:

I want it to be fluid at very low RPMs (0-20). I was thinking of using a stepper motor that way I can control the speed and the rotation. However, I am not sure which configuration to use; spark fun offers two stepper motors (a 200 step and 400 step motor) and two drivers (one with microstepping down to 1/8th and one that goes down to 1/16th). I was planning on using the 400 step motor with the 1/16th microstep however its the most expensive option and I am not sure if its necessary.

stepper 400: stepper 200: driver 1/8: driver 1/16:

Without looking at the product links, I would suggest that you go for the 200 & 1/8 (would that be 1.8?) setup. Every step takes 360/200 degree (200 steps pr round) = 1.8 degrees pr step. The micro stepping may have a higher torque, but it doesn't seem to be needed. There are a couple of other concerns, as ie 'noise/resonance' but you can't really deal with it on a link to a product. I don't think that the choice is of great consequence. Chagrin's suggestion in your other thread has a really good point though.

Resonance can be a real problem - microstepping definitely helps prevent mis-stepping due to resonances, as does mechanical damping. Microstepping will definitely help keep the motion fluid, I suspect it will be worth it. Go for 1/16 stepping and the cheaper motor perhaps? 200 steps per rev is standard, I'd go for that and shop around for the cheapest suitable motor. At 20rpm you may not have to worry about resonances though - might not be high enough stepping frequency to trigger a resonance (that does depend on what you are driving mechanical though). A bipolar microstepping driver will handle 4 6 or 8 wire motors. 5 wire motors should be avoided.

I don't know anything about rc servos; what advantages do they present that a stepper doesn't? the rotation can be programmed into the stepper so I achieve any angle I wish and by incorporating delays I can adjust rotation speed. the only difference is fluidity however a high step stepper motor seems pretty fluid