For a round diamond, I prefer 6-prong. Holds the diamond in better, and keeps it actually looking round. In my opinion, 4-prong ends up looking a little squared, and I'm not a big fan of that.
And if you are paying for a brand name, I'd go with Tiffany over Cartier, just because it's SO well advertised for engagements, the "little blue box" is something people dream about. Don't get me wrong, I'm sure they are great quality, and probably really take good care of you in the store (they better). Even though I'm not generally a big fan of expensive jewelry/diamonds (beautiful but industry is kind of

), I can appreciate how beautifully designed and made Tiffany jewelry is.
So, that would be my vote. But obviously the most important is what you think she will love/be excited about--and want to wear forever.