Newbie Question

I've decided to "roll my own" minimill cnc conversion rather than buy
something already built to save some $$$.

I will be using stepper motors. I am trying to decide on a
controller. I have been seriously considering the Xylotex paralell
controller and the deckcnc serial controller.

#1 What are the advantages and disadvantages of these controllers?

#2 Are they in some way separated from the computer to prevent damage
in the event of a mishap?

Thanks in advance for your response.

Jack