I still think Dolby Headphone is the Xonar's big advantage for gaming over X-Fi.
You can ger round this. Just get a DDL capable X-fi (including the Forte but there's no need to spend that much if you're using digital out) and use a Dolby Headphone capable AV amp/receiver (effectively using an external DAC) with a good headphone amp. In my opinion, this is the best possible, money no object, solution.
Given that you game with headphones, I would say go for the STX, unless you have a Dolby Headphone capable receiver.
X-Fi is the way to go if you're gaming with speakers. Xonar STX is great as a standalone option for gaming with headphones.
I wouldn't worry too much. They're both great soundcards. I don't think you'll be disapponted with either. I really enjoyed trying out an X-Fi Prelude. If I could have got Alchemy to work properly with both cards installed, I would have probably kept both.
Since I tried both, Xonar drivers have inproved for gaming. I'm a lot happier with it now than a couple of months ago.