Really they do not CERTIFY. No lab does that. Many labs grade diamonds and make REPORTS which include GIA and EGL.
There are differences between labs. Some are more liberal than others. Some give you more data and some of that data is not only important but reliable, The GIA gives very little data, but is highly respected for grading accuracy.
You have a large menu of service providers and each has their own strong and weak points. It is no simpler than the question of who to buy a diamond from.
US dealers tend to use GIA for accurate grading of fancy shaped diamonds. EGL is an alternative for some. It is less costly, and a bit faster in getting the work done, too.
In some sense you get what you pay for. Sometimes a bargain is not what is exactly cracked up to be. Dealers use EGL becasuse it works for them. Will it work as well for you as a GIA report. That depends on your own judgment, your budget, and how thoroughly you shop and learn about the subject of diamonds.
I plan on coming to you once I make my decision-LOL But one more question. Know alot about rounds because I have been educating myself and saw your stats on ovals but there doesn't seem to be as much info on ovals and pricing. Also, I seem to like the ovals at the higher end ratio @1.5:1-1.66:1 to give a thinner look but seems most are at the 1.33:1 range
Ovals are cut from material that would not always cut a good round or a long marquise shape. Kind of moderately long rough but not long enough for some other shape. That's part of the reason most are shorter in l:w ratio. Of course, mother nature plays a part in what rough is available and personal preference for what makes an "oval" shape is part of this consideration. The fact that you like longer ones has no effect on availability or supply... We always seem to like what we cannot find, don't we?