Wireless is good. But sometimes there are environmental interference, or light issues if you're doing Infra-red. That's where a nice long, straight cable is invaluable to have in your bag. I carry a Flashzebra 24' cable I bought a couple years back. Since then, Syl Arena has come up with OCFGear and they have a 33' cord I'd like to get my hands on. An extra 9' would have come in handy on a couple occasions.
I prefer the straight cables. Why not buy a 33' coiled cord, they're out there. I'll steal a quote from Syl Arena:
The advantage of our straight cords over long coiled cords is that our cords will drop to the floor rather than swing through the air. So, the tripping hazard is reduced. Also, it takes little effort to extend our straight cords out to length. With a coiled cord you have to pull firmly to get it to stretch — which will likely topple your light stand. Coiled cords are great for arm’s length work, but not for long distances.
To be honest, I can't see going much more then 3' with a coiled cord. With a 6' or 7' cable, you might be tempted to stretch it and get it the full distance away which is just disaster waiting to happen. If you're not holding the flash, I recommend a straight cord and the uses of gaffer tape. A few well placed pieces of tape can preserve your flash and gaffer tape is easy to move and reuse.
Before buying, I would make sure the cable is compatible with your flash. Some Sigma flashes needs a more powerful signal and won't work with some of the longer cables.