zlacker

I like flat laces a lot because they seem to distribute the force a lot better than round styles. I like my shoes to be fairly minimalist, I don't like overly padded or heavy shoes, and round styles seem to put a lot more pressure on the top of my foot.

The oval style with the sharp edges has a lot of the benefits of the flat style, but is more universally compatible. The flat ones don't always move well in round eyelets, they work better with loops in my experience.

The completely flat style doesn't tie well if one of the laces is twisted though. I think it's a great option for athletic shoes though, where you are more likely to care about a particular level of give, etc, and when you don't want your shoes to become untied. The round style is cheaper, and if you don't notice the difference it's probably a more versatile shoelace.

Plus you can do fancy knots with the round style.