ROG Xonar
Audio Processor :C-Media CMI8888DHT High-Definition Sound Processor (Max. 192kHz/24bit)
Features
Dolby® Technologies : Dolby® Home Theater v4
Smart Volume Normalizer™
Xear Surround™
Magic Voice™
FlexBass™
GX 3.0 Game Audio Engine
Xonar D2X
Audio Processor :ASUS AV200 High-Definition Sound Processor (Max. 192KHz/24bit)
Features
Dolby® Technologies : Dolby® Digital Live/Dolby® Headphone /Dolby® Virtual Speaker /Dolby® Pro-Logic II
DTS® Technologies : DTS Interactive Encoder/DTS Neo

C technologies
Magic Voice™
FlexBass™
Smart Volume Normalizer™
GX 2.0
Dolby and DTS for Complete Digital Home Experience
Besides delivering impressive audio quality, the Xonar D2X is also packed with multiple sound technologies from Dolby and DTS to deliver outstanding entertainment experiences. These include
Dolby Digital Live: Converts PC or game audio content into Dolby Digital in real-time
Dolby Headphone: Creates a surround sound listening experience when using any set of headphones
Dolby Virtual Speaker: Delivers a vibrant surround sound listening experience from stereo speakers
Dolby Pro Logic® IIx: C reates up to 7.1-channel surround sounds from stereo or 5.1 sources
DTS Connect: Includes DTS Interactive and DTS NEO

C
DTS Interactive for real time AC-3 encoding and
DTS NEO: PC stereo to multi-channel expander
From what I can see it comes down to what has the better Audio chip. The D2X has DTS, the other Dolby.
ROG supports GX 3.0 vs 2.0 I found this to be more trouble than its worth on my D2X.