Running steppers series v/s parallel

My new 600in/oz motors came in today, happy, happy..
SPECS:
8 wire bipolar motors
4amps 3.6V parallel (faster/less smooth???)
2amps 6.8V series (slower/more smooth???)
The question is:
Do I run them in series or parallel?
and what effect does this have on smoothness/speed/microstepping.
Any opions on how to wire them for a mill/drill (series v/s parallel )
They will be run from a 68V 10A power supply.
the driver will be a geckodrive g201a.
And lastly what is the differnce between g201a and the new G201 rev5.


Thanks in advance.
Jerry