The latter (Extreme Audio) isn't actually a X-Fi card. It's an Audigy rebranded as X-Fi serie product, enabling some X-Fi features using software.
When it comes to X-Fi based products, Auzentech actually has better products than Creative itself has. Partly due to quality parts used on the soundcard. But Auzentech is more premium priced: 140 euro for Auzentech Forte that offers the best quality when it comes to X-Fi based product that's focused on gaming.
If you are using your computer more for music and movies than for gaming. You could also consider Asus Xonar DX (PCI-e) if you wants bang for bucks audio quality improvements (somewhere around 70 euro).
The Xonar products uses DS3D GX to emulate EAX (1,2,3,4,HD) where as Creative uses Alchemy to re-enable EAX for games (but not every game with EAX is supported actually) in Vista/Windows7. Most games is moving over to OpenAL API which works natively on Vista/Windows7 without the need for a hack like Alchemy. EAX is going to slowly disappear from the gaming landscape. So you decide how important EAX is going to be for you.