it depends more on the overall quality of the cable than the shape of it.
You can take a good cable that transfers data well, and make it into a round cable, and it will STILL transfer data well.
Or take a crappy cable that transfers slow, and after rounding it'll still suck.
But if you compare the quality flat cables with the poor quality rounded ones, it would seem that rounded cables suck! This is what many of the websites that review such things are doing. Plus, with round cables being so popular, companies are trying to score a fast buck by making them very cheaply.
In otherwords- get round ones if you want, just don't skimp on them. There is no performance difference inherent in the shape of the cables.