I've seen a lot of slings on here with flat braids and have made one myself with a flat braid. But I've found it causes massive air resistance, probably due to it being flat and catching the air. As to round cordage it seems like it cuts through the air better. So what are the reason for flat braids being used in many slings? And what are the benefits/problems with flat and round braids? Thanks! 

I used flat early on simply because a 3 strand flat braid is easy to make. But I prefer round. So long as the cord isn't overly large either one works ok.

This is very interesting. Someone should do a little experimenting on the subject? 

That would be Mas.  He really is a rocket scientist.

But, air resistance is a function of surface area.  When you are figuring surface area, you have to include total surface area.  That means all the bumps and pockets between strands will have to be figured out for the total surface area.

Or you could just use a leather pocket and para cord. Or a woven pocket with para cord.   

And if you want really low air resistance, use Kevlar fishing line.  However Kevlar doesn't really handle abrasion very well, so you will need to watch that.

All that being said, unless your cords are thicker than your thumb, you probably won't notice any difference.