I've only ever used Milliput, just because it was always available in hobby shops when I was younger and now because it is what I'm used to. From reading, Green Stuff stays a bit more flexible, and perhaps has a bit less of a rough texture - would that sound about right?
Also, I tend to do most of my detail work when the putty has hardened considerably (more used to carving wood really); would that be a reasonable approach with Green Stuff, or should it be done before much of the cure has progressed?
I feel like I ought to try it, given it seems to be preferred for miniature sculpting!