I vote for the figure 8 necklace. I love the heart design (and constantly steal my sisters heart earrings lol) but I think the figure 8 is more unique.
I do like the figure 8 necklace and feel that it is the less common of the 2.
Personally, though i LOVE the open heart and have both the medium and small pendant and wear them on a daily basis just changing the size depending on my neckline.
I love the "Open Heart" but I think you'd have to go and try it on to see for yourself if you like it or not. I have a slightly wider neck so I know for sure the "figure eight" necklace wouldn't look pretty on me.