Technically speaking, the Revolution from M-Audio is a superior card to the Audigy 2. It's only superior by several minor levels that any normal human being (read: NON-Audio Engineer) would never notice. It also supports Dolby 7.1, and full 192khz playback on all channels even in surround mode unlike the Audigy (once again, not like you'd notice)
However, on top of it having slightly higher specs than the Audigy, it also retails for $30 less because it doesn't have unnecessary things formost consumers like firewire and S/PDIF outputs on it.
Also, there are several well documented cases of incompatibilities with Creatives drives under specific MB chipsets and Windows Mr Smug and obnoxious.
More importantly, Creative has developed a nasty habit of not making new drivers available for their hardware as soon as they have a new card line available. It was quite difficult to find drivers for the Live! series cards on their website for several months when Audigy came out because they felt that everyone should buy their new product.
Found a nice pretty link giving an example of Creative driver problems...
http://www.viaarena.com/?PageID=26BTW, just to make a quick point, I think the Audigy 2 is an excellent card. Creative makes lousy drivers however, and the company has a habit of trying to rip off it's consumers as much as it can get away with IMHO.