Hi Peter,
The Open DMX has stability problems and the fading is jolty sometimes. Something to do with not being able to store an entire DMX frame at the same time I think. I read an article a while ago that explained the exact differences, but I can't seem to find it again.
I have had no issues with the Pro at all. So definitely go with the Pro if you can. It isn't that much more expensive anyway, and it just doesn't break.
David Kirkpatrick