If you are running the card in the proper mode(all setting properly set), then they will both sound identical to human ears. There is no credible evidence to suggest otherwise.
BTW, I highly recommend running the Audigy 2(if you get it) in 96khz mode. Use re-sampling in Foobar to play 44.1khz files at 96kHz. Otherwise, the built-in resampler in the Audigy may produce undesirable artifacts. The Xi-Fi cards have superb resamplers, so it's not an issue with that.
-Chris