Such an interesting question, and such great visuals! I'm not a big jewelry wearer, but I have an opinion (of course!). To me, inside the collar feels more modern because it is less expected. Outside feels more "appropriate" somehow, more conservative, more traditional. That might make "outside" the rule, and "inside" the rule-breaker.
After seeing these pics, I myself am inclined to go inside. I love the pic of the woman in the white blouse with the gold necklace inside. The lovely line of the blouse is not interrupted, and the necklace drapes out so beautifully. Love that.