Audigy has considerably lower CPU usage, even with many, many streams. It also has those nice 1394 ports 😀
Seriously, there isn't much of an incentive to upgrade if you already own an SB! Live 5.1. Unless you've got a killer sound system, you won't notice a bit of difference. Then again, with OEM Audigy's in the $60 range, you really can't go wrong. The choice is yours...