You say 'best sound quality' but do not state what sort of source file you will be attempting to play back.
The Dragonfly is good up to 96K (even though it has an ESS Sabre DAC which is a bit strange) and the Meridian will handle up to 192k resolution. I believe the Meridian is more likely to be able to handle more 'difficult' headphones should you ever upgrade yours. Whether or not this is sufficient to pay out the extra £70 is up to you. I have not heard the Xonar so cannot comment on this device.