For pure sound quality I would say it's down to preference. The higher quality audio components in a soundcard or DAC should be noticeable on the Aego Ms but improvement can be subtle.
For features I would say it depends on what games you play. If you play current games using OpenAL or older games with OpenAL / DirectSound3D an X-Fi is pretty much the best option. The hardware features can really enhance the in-game audio simulation. A Xonar can do well using it's DS3D-GX function but results vary considerably. In some games it's pracically as good as X-Fi. In others sound problems or instability is apparrent, forcing you to turn DS3D-GX off and being left with something worse than many onboard solutions.
Many games now use a software only engine now though, where all effects are processed by the CPU. They usually have no discernable difference using different sound devices.
Some games use FMOD, which is a primarily software engine that allows hardware DSP effects to be utilised. Here an X-Fi or Xonar can sound very similar (assuming DS3D-GX works) and are the best options.
For OpenAL, there are some viable software alternatives to a soundcard. BlueRipple do some software that scales with available CPU resources. Given a fast quad-core it can technically do a better job than X-Fi. It's offers no benefit for games using software engines or DirectSound3d.
For headphone use most games have audio intended for speakers. Here some soundcards include headphone virtualisation, that allows the game to simulate surround speakers using headphones. Dolby Headphone on Xonar is my preferred option for this. CMSS-3D Headphone on X-Fi is similar but doesn't sound as realistic IMO.
Some sound devices also have speaker virtualisation, that allows different speaker configurations to simulate surround speakers. These may be relevant to you with the Aego Ms but in practice you need to set up your speakers to make sure you're in the sweet spot and room acoustics also need to be favourable for it to work.
For both headphone and speaker surround virtualisation there's a chance that it just doesn't work for you.
If you play old (Pre Vista) games using DirectSound3D, Creative cards including Audigy and X-Fi offer the best support via theirAlchemy software. Xonar, C-Media and Realtek all have some form of support that can be a bit hit-and-miss. Without these you will be restricted to stereo / software only mode that does't take advantage of the sound processing capability.
Be careful about exact sound card models. The X-Fi Xtreme Audio, for example, is based on older rebranded hardware and lacks the advanced gaming features of some other X-Fis. Likewise, the Xonar DS lacks Dolby Headphone and Dolby Virtual Speaker that are available on most other Xonars.
