Since i'm in the same "position" as ericB... i will attempt to answer that question too.
I have not physically HEARD the X-fi, but i dont have to... as an engineer, i was trained to be able to read spec sheets and know what things (mics, speakers, etc etc) will sound like before even seeing them. I know that it will sound the same as the Audigy 2 ZS.
The X-fi does have better EAX support. I will give it that. BUT... here is something to think about. When directX 9 came out... could you REALLY REALLY tell a difference between V9 and V8? I doubt it. Same applies here... can you REALLY REALLY tell the difference from EAX 5 to the older version (whatever it is now... i dont remember)??? You are still getting the same audio signals to your speakers... what does it really matter?
Having an audigy 2 ZS may make people think that I am biased. I am not exactly biased... I love creative's hardware and both of the cards are from creative. I dont like the fact that they lie about the much better quality when it is nearly exactly the same card they already have... just with an "optional" crystallizer that will be turned off anyway (at least by smart people).
If it came down to that, I, being an audiophile, would closely A/B the two cards. Whichever sounded best (which probably wouldn't happen since they are virtually the same card) I would choose.
I do think its funny that since the X-Fi came out, the audigy series' price went way up. I bought my ZS for $110 retail, no sales, no coupons... original price @ circuit city. Now check the price... $130-$140. Why do you think that happened? X-fi didn't get as big as they hoped... so they dropped the price of the X-fi and raised the price of the audigy to make up for it.
Nice...
EDIT: For those of you comparing the audigy and X-fi with the same speakers: you MUST MUST MST have them in the same room, same position and on the SAME platform/desk. If not, then of course they will sound different. If you took an amp and a sub for a car and put one set in a car and the other identical set and put it in a metal trash can, would it sound the same???? HMMMM!!!??