There won't be that much of a quality difference between the two, but the SB Live! will generally tax the CPU less in doing audio processing. Some on-board sound solutions also don't tax the CPU that much either... I'd rather go with that than the SB Live!.
And again it also depends on your speakers, it's exactly like video cards and monitors to a degree. What does it matter if you're getting 200 vs 300 FPS if your (CRT) monitor can only handle a 60hz refresh rate? Speakers is usually what limits people as far as audio quality. The other thing to consider, again, is CPU utilization.