Like Heyyou27 said, what do you want to use it for?
Also I believe the Audigy 2 Plat is around $130, while the X-Fi ExtremeMusic can be had for around $70. There's quite a price difference.
In terms of noise levels they should be very similar, since they use the DACs (Digital to Analog Converters) although surprisingly the X-Fi has been measured with higher (worse) noise levels. These are measurable but not audible quantities. You won't be able to hear the difference between -90db and -100 db noise levels.
If you use the analog interconnects, the X-Fi has a slight advantage in that Creative spent a bunch of money developing a processor whose primary purpose is resampling of all input sources to 48KHz. Both cards resample; and thus this does add distortion to non-48Khz input sources, but the X-Fi does it better than the Audigy 2.
The X-Fi has a totally different software package. This includes things like the Crystalizer and CMSS3D-Headphone. If you are an adventurous sort, you could hack this to work with the Audigy 2ZS, or maybe its worth more to you if you paid for it instead. The Crystalizer is an EQ the shape of a smily face. If you have speakers that struggle to accurately reproduce input sources, this *might* help, or might make it worse depening on the source. If you have decent speakers, this might not really be worth much to you, especially since its a one-size-fits all EQ whereas you could tailor something that fits you better. The headphone thing I haven't seen any measurements or reviews for. So far its been metaphorically speaking, vaporware (in the objective review department).
The ZS Platinum has an external breakout box which has features you might like or not. Figure out for yourself which is more important.