The Audigy2 ZS has considerably better DACs with higher signal/noise ratio,
higher sensitivity, lower distortion and such, but wether you'll hear that
in the end is debatable if you're encoding the sound to a "lossy" format
such as MP3 anyway.

MP3 varies from awesome to total crap depending on
the encoder you use and the parameters supplied to the same. Also, the
equipment you'll use to play back the sound matters. If it isn't of
particulary high quality, it won't matter even if your soundcard delivers
completely flawless sound, you won't hear the difference anyway.
Anyway, I have a ZS, and I am very happy with it. I think it is a very nice
sound card, and the generic version without the I/O break-out boxes isn't
particulary expensive either.