Diamonds are a commodity, independently certified by GIA. Really, high end brands exist because they generally have a wider selection and more high end diamonds.
I went to Tiffany only to discover their diamonds weren't good enough quality for my standards and they were unwilling to make me a ring from a loose stone. I went with Blue Nile and I'm glad I did.
I ended up with a gorgeous E Color, VVS1, Signature Ideal cut, around 1.5 carat with no florescence, and excellent polish and symmetry. It's laser inscribed with GIA certification, and also certified by AGSL. It's set in platinum...simple, but stunning.
"You can't tell the difference" is just salesman talk trying to sell you a lesser stone...I know it's an extremely rare stone and that's what I wanted for my wife. It's true, you can't tell the difference by looking, but my wife's E color is absolutely colorless and nearly flawless It really does make those Jared and Zales diamonds look yellow when other girls compare rings.
Be ready to pony up though...quality diamonds aren't cheap. Sure, I could get more bang for my buck, but I wanted a nearly flawless, colorless stone.
Blue Nile has everything and great customer service.