Hi Kev,
Well those two options are quite different. The Xonar D2X looks cool and has surround sound, if you need that then it's most likely a good idea to get an internal card unless you just output optical to a decent AV amp. If stereo is all you need and care about then definitely get the rPAC, I can highly recommend that as a portable (with a laptop) headphone amp as well as a DAC, but yeah it will also output an analogue signal from the RCA connectors at the back to a stereo amp to great effect.
It's worth noting that the rPAC doesn't support 24bit 192khz (only up to 24bit 96khz) but I wouldn't let that put you off unless you have a ton of music in the former format. Purely down to quality of DAC the rPAC will win because it's only stereo. I think the Xonar will need at least 4 DAC chips for it's surround output and thus the quality of components in the rPAC will be much better and much more focused (more simple) for stereo quality.