Generally speaking there is no performance difference. Some people say they get data corruption more frequently with rounded cables, but IMHO this is more likely a metter of poor quality cables. A bad cable, rounded or ribbon is a bad cable. I've been using rounded cables for at least a year, probably more, and have never experienced any data corruption problems on any of my drives.
Now that I think about it, I can make arguements for the "tidyness" of either type. In my server rig, I have a mix of rounded and ribbon. The ribbon cables connect two of my hard drives to the PCI Controller card. Ribbon cables were much more tidy here, because they could be folded and laid flat on the floor of the case. I found rounded too rigid for this purpose. The rounded cables work for my optical drives due to the placement of the IDE connectors on the motherboard. Ribbin cable origami would have been impracticle here. I do admit to liking the look of rounded cables over the ribbon cables, unless they are folded super schweet.