Skip them both and go parallel.
I've had a GRex for several years waiting on spindle sync for threading on a lathe. It just hasn't happened despite being promised many many times. There's a bit of a tussle between various parties over whether it is a firmware (GRex) or Mach 3 problem, and in the end of the day it doesn't really matter. The users don't have the feature.
Then the news about issues with moves for mills came out on GRex, which it seems really limits the advantage there. Meanwhile, the pulse rates are way up on Mach and it seems that the parallel port remains the best performing and best all-round platform for Mach 3.
Maybe ncpod and these others things will be better, but what I learned from the experience is go with what the vast majority of the user community has and don't be on the bleeding edge. Artsoft just doesn't really have the manpower to support things on the edge, and neither, apparently, does Gecko.
Both companies make excellent products, but stick to their power alley!
When one of these other things has been out for like 2 years and all the problems are solved, then maybe I would consider one.
YMMV, just one man's opinion.