Pricescope is good. The Rocktalky forum has good advice on not only what makes a very good cut, but selecting between the various colors and clarities. Be aware that many of the on line vendors are selling from wholesale lists that they do NOT own, and it many cases have not even seen. Others have in house diamonds that they have selected for their brand. I shopped locally - nothing but low quality and high prices.
Laboratory diamonds are more and more available and De Beers just announced that they will be undercutting all of the current lab grown prices as soon as their factory is up and running in Oregon. They're talking $800 per carat, with limited selection and no stones over one carat.
If you pick a vendor that knows where they source their diamonds you can ease your mind that you're supporting some dirt poor African who is supporting his family by scratching in the ground. There are numerous choices from around the world.
Me? I bought an engagement ring in April from Whiteflash.com in Houston. One of their so-called Super Ideal cut round stones. Pricescope has info along with a variety of diamond related blogs that pop up with a google search on super ideal cut. Cut is king; that's where you get the sparkle, flashes of fire and all the good stuff. You don't need the colorless diamonds to look good, only a great cut.
I'm retired. I could afford it. She's happy and everyone loves it and the positive comments keep on coming.
I had this diamond www.whiteflash.com/loose-diamonds/round ... 955177.htm
set in a platinum Vatache U-113 (similar to the famous Tiffany 6-prong setting.) For those who don't want to click - A 1.514 F VVS2 round. And the flashes of color in the video are easily visible in real life.
It's only money.