I prefer the Cartier - I have the rose gold with pink sapphire love ring--and I, well, love wearing it. I do like the shape of the Bvlgari, but I don't like the branding on the side, the design already speaks for itself. However, I would go with the Cartier if I had to choose between the two. I myself also like the ones with 1 or no diamonds b/c I like to see the screw motif--that's the whole point with the love collection.
Both rings are beautiful, enjoy the one you choose!