If your main interest is purely in gaming you could save a few dollars and go with the Audigy 2 but if you're a bit of an audiophile and/or musician and good sound (for recording, CD, mp3, DVD playback, etc) is also important to you go for the Audigy 2 ZS.
Here's a
review/comparison between an M-Audio Revolution 7.1 and the Audigy 2 ZS which also goes into some details about the differences between the older Audigy 2 and the new Audigy 2 ZS. According to the article the differences are mainly to do with audio quality - here's an excerpt:
"Purely in terms of manufacturer's SNR numbers, the Revolution 7.1 sits between the original Creative Audigy 2 (which claimed an SNR of 106dBA and 6.1 playback) and the newer Audigy 2 ZS (with an SNR of 108dBA and 7.1 playback). Both the Revolution and Audigy 2 ZS have similar recording capabilities, with support for genuine 24-bit/96kHz recording with low-latency ASIO support (the original Audigy 2 was limited to 16-bit/48kHz ASIO support)."
The difference between 106dBa and 108dBa SNR is roughly equivalent to the difference between, say, a top-of-the-line Kenwood CD player (Japanese) and a mid-level Denon (British) and the original Audigy 2's limit of 16-bit/48kHz ASIO support was a serious fun killer for many musicians.