Much better to get a stereo set with either of those soundcards. X-Fi has CMSS-3D headphone. Xonar has Dolby Headphone. These both simulate surround sound over headphones. I have a set of Speedlink Medusa 5.1 and simulated surround via those soundcards on stereo headphones is much better than the multiple driver 5.1 headphones.
I have both a Xonar D2 and an X-Fi Prelude. The X-Fi is marginally better for gaming generally but the Xonar's EAX5 emulation is pretty close. You get marginally better effects in EAX titles and slightly better positioning in all hardware accelerated titles on the X-Fi. There have been occasions when I've had to wait a long time for Xonar driver updates before getting a game working properly with EAX. With X-Fi, it usually works and if now I can usually get things working manually before the drivers are updated.
The main gaming problem I had with Xonar was older EAX 3 and 4 titles not working properly (so you have to use EAX2). Not sure if they'll ever fix this.
The reason I asked about headphones was that different headphones have better synergy with different cards. Headphones with a wide and forward soundstage (like Sennheiser HD555, HD515, HD595, Audio Technica ATH AD700) tend to sound very good with X-Fi. Headpones with a more neutral soundstage (Goldring DR50, DR100, DR150, NS1000, Sennheiser HD580, HD600) thend to work very well on Xonar.
Edit: I should mention that for headphone use, I much prefer the Xonar implementation even in games (at least on my headphones). I only have the X-Fi installed now because I have an external Dolby Headphone processor (Victor SUDH1). If it wasn't for that, I would still have the Xonar installed in my main gaming machine.