The Xitel has a reputation for recessed bass. In part it is deserved, but the real problem is that the crappy cables that ship with it have recessed bass. Used with different cables, the sound is quite good. Also, the Xitel has both optical and RCA digital out, and it uses the standard OS level usb audio driver and thus works on all operating systems (Linux works great though FreeBSD seems to be a bit flaky.)
The sonica (I own them both) only has optical, though it has a better analog section than the xitel. The digital sections are pretty equal in quality though the sonica is a bit fatter in the bass. Also, the m-audio requires the m-audio drivers which can present a hastle.