The Extigy is the newest version of the Audigy. It's just an external based version of the Audigy. They both perform the same, but the Extigy has a wealth of different plugs, like optical, IEEE1394/Firewire, USB, SPDIF, MIDI, etc.
If you can afford it go Extigy, the Gamer and MP3+ versions of the Audigy are half as much as the Extigy, at least I think. Prices may have dropped.