The Fiio E10's headphone amplifier is low impedance, I'm guessing the Xonar DG/DGX is not low impedance
As long your ok with 2-channel stereo audio, then get the E10.
If you want surround sound for movies or gaming, then the Xonar DG with an amplifier.
You can always try the Xonar DG first, without an add-on amplifier, see if you like the sound.
If you went with the Xonsr DG(X), what would you budget for an amp?
Just under $100 for both the Card and AMP.