There's a big debate on this.
Bio-balls can become a bit of a nitrate factory.
If you have a Fish only tank (that's not overstocked), and do frequent (every other week) water changes, you probably won't have a problem. But Live Rock is still a much better route to go.
The general rule for Live Rock is about 1 to 1.5 pounds per gallon. It depends on how pourous the rock is. Smoother rock you'd need more. Really bumpy rock, you need less. I generally shoot for a little over a pound per gallon.
I definitely recommend using Live rock. Much more efficient, I think it's prettier to look at, and MUCH more interesting to watch. It also buffer's your tank from mistakes MUCH better than bio-balls can.
Please note though, that if you add Live Rock to your existing tank, it will cause another cycle that can be fatal to your current inhabitants. You may need to either cure the rock in a tub for a few weeks, or move your fish to a temp tank.
Here's a link to a pretty good discussion about Live Rock and Bio-Balls, and filtration in general. I really recommend you read it before deciding:
http://thereeftank.com/forums/showthread.php?t=51281
