Surfergirl, I will say that the certificates and stats on diamonds while important, isn't THE most important element. YOU have to love it. The thing with Tiffanys is that ALL of their diamonds are high quality- you as a client are paying for that after all. Tiffanys isn't going to sell sub-standard stones.
There are some women who only want clear, flawless, colorless diamonds......anything below a "D" is unacceptable. My current diamond is gorgeous and surpasses the quality of my former cushion. But you know something? My cushion had character! Even though it was a superior cut etc.....and was F in color, it also emitted these rainbow-like flashes which I really, really miss! When I look back at the photos, I can appreciate that now.
In my opinion, a diamond has to have character, sparkle, and I have to love it because I AM the one who will be looking at it ALL THE TIME.
Personally I'd stop obsessing about the stats and just go there and SEE which one you love. You might surprise yourself how easy and effortless it will all become