The X-Fi is a completely different Soundcard to the Audigy 2/Audigy 4 range. So, errm, are you stuck between the 2 Audigy's or is X-Fi is an option for you?
Just from personal expirence. I have a Audigy 2 and i have had the chance to listen to an X-fi.. let me tell you, i hear a huge difference. the x-fi is great..if you have the money