I asked someone who keeps both species to check out the differences on really tiny nymphs. Here is his response:
"Well, they really do look very much alike.
The only difference my untrained eye can tell is that the discoid nymphs have a more rounded, crescent shaped pronotum than the dubia's. The overall body shape also seems to be more teardrop in the dubia's, but this just may be due to the age differences between first shedding and primary (newborns)."