So the Ethernet looks to be the best then from a practical standpoint, is there any performance benefit for pci over Ethernet?
If you a thinking linuxcnc then there is no difference in performance but if you want a longer cable from the pc to the control box then Ethernet is the winner.

I use Ethernet with Mach3 to a Russian controller on the router.

I have no experience with the UC300ETH so can't comment.

You will have to think if threading is important and if you want to control the spindle speed to be able to use G96 CSP like;- http://www.mmsonline.com/columns/the...-surface-speed

Being able to use diameter mode or radius mode etc. and ATC need to be considered if you so wish.