Oh that's a beautiful necklace
Personally I really like the white background, I feel it really brightens the whole picture and seems much more modern and sleek and for me makes the jewelry really pop.
i second this (also on the prettiness of the necklace :--)). the white feels more professional and luminous. the taupe backdrop gives me a first impression of a shadily lit backside of a wardrobe, which obviously isn't the look you are after. though i guess that the fact that the orange is more orange on the pic on white impacts on this too..
though you only asked for opinions on the background, i'd say you might want to give the top edge just a bit more empty space above the dummy's neck. it'll make the pictures look more balanced. the same as on its left and right'll do. the need for space is more pronounced on the darker background, the white one isn't way too tight.