I'm only concerned about it's gaming sound and performance, I don't care for music, HTPC, recording or any thing in relations to that.
The primary game this will be for is Lord of the Rings Online, with occasional CoD 4/5, Supreme Commander. I had enough of Creative, still getting BSoD after so patches, they never fixed this. I'm sick of the lack of Realtek support for the game, and they still only have a very old beta driver to restore 5.1 sounds, and the sounds are not stable, they keep peaking up on me, even when I turn the ingame sliders down.
I have a Striker connected to a 5.1 system, and I'm really happy with it.
The ASUS also receives good reviews, but I haven't heard it myself.
One strange thing I find with the Striker, is I have to open the sound utility and send a test sound to one of the rear speakers to 'initialize' the 5.1 sound. If I skip this step and go directly into a game, I only get 2 channel sound.
Crysis, HL2 and many others just would not be the same without hearing which direction the incoming fire is coming from.
I assume you game with 5.1 speakers? In which case, i'd go for the Xonar if I had an extra floppy power connector.
In terms of FPS, benchmarks show the fatal1ty sound card to be better than all the other ones (by 1-5fps). The Xonar has the better sound quality, again according to benchmarks, but it is slightly less convenient to use because of that extra power cable requirement.