My experience with round cable is positive. I use Cooler Master cables (high quality in my opinion).
Some suggest do not use 24 inch cables (for hardware reasons, don't remember the exact site where I've read that) but only 18 inch cables. Sorry: haven't try any 24 inch cable so I can't refer experiences about it.
Regarding better air flow, my comp is such a mess that there is no difference with a flat cable, but assembly was certainly easier.