You have great taste in soundcards. Either of these card are kickin' audio products but they are meant for very different tasks. The Auzentech Prelude is more of a gamer card and supports gaming features such as high levels of EAX and openAL. The Xonar is meant more as a HTPC card and lacks the gaming features. The weak part of the Xonar is the 45XX series opamps on every channel except the fronts which sport a LM4562 amp. This card doesn't lend well to being modded the opamps are hard wired to the board.
You could swap out the 45XX series for better opamps such as LM4562's -if you go with the Xonar that is... Either card sound amazing without any modding needed. COnsidering both are worth $200, make sure your handy with a soldering iron as $200 is a bit much to pay for a paperweight.

If you have any mopre questions, feel free to ask. Good Luck with your purchase.