I recently got the Cartier Diamants Legers in white gold, size L. Love it and wear it 24/7, except when cleaning. I chose it over the DBTY because I prefer how sparkly the chain and bezel are on the Cartier. When the light hits the bezel right, it really makes the diamond stand out. I also like the Cartier dangle clasp. The necklace has only spun around once and my husband thought I was wearing a new necklace. That said, I wish the diamond sparkled a bit more, although it's very white and clear.
I think if you want the focus to be on the necklace as a whole, go with Cartier. If you want the focus only on the diamond, go with Tiffany.