First: both are the same species, Arothron Meleagris. Officially just different color morphs of the same species. Pure "Guinea Fowl" specimens are black with white spots in regular pattern/intervals all over their bodies. There are also pure golden ones that are a golden color with little to no spots (or the ones they have are randomized). There are also specimens that seem to be in between and appear sort of "banana turd/spoiled banana" looking, ie black with significant sized random patches of gold and still having the white spot pattern over most of their body.
From extensive conversations with importers, wholesalers, a professor of marine biology, and various hobbyists the hypothosis (nobody has a hard answer yet) is that the golden phase is actually male while the guinea fowl stage is female. If anyone has anything concrete on this I'd love to read it!
However, it also seems that in 99.x% of the cases once the animal is in captivity it does not change color (or thus possibly gender?!) That isn't to say they are not beautiful and personable in their own right. Just don't buy a small guinea fowl (black w' white spots) and expect it to turn gold the way a red coris changes or anything.
I have owned both and love my golden. I'm attaching pics of both the current golden and the guinea fowl. (adopted her from an LFS where she was a rescue case, ie somebody dropped her off emaciated and beat to *&%#) Regrettably she would never eat but was so friendly she would swim onto your hand or brush it if you put it in the tank with her. Pics below:
Hope this helps...
PS if anyone has any further info on this topic please let me know!