I like the Cartier, but the Tiffany is such a classic. It is simple, streamline, and easy to find a band the goes with it. I would imagine that the Cartier would be hard to by a band for unless you get the exact matching band.
My vote goes to Tiffany too I would also drop IF to VS1 or 2 to get a better colour with 3 EX...you can also check if the diamond is IDEAL CUT using the online calculator.
Tiffany does sell ideal cut diamonds, but you need to do your homework and be assertive in asking what you want/ having other rings transferred in.
My engagement ring is a Tiffany (but the classic three stone) and I'm very happy the quality and routine service. I think you can drop to VVS clarity. My stones are VVS and I get compliments on how perfectly clear they are all the time.