I've only ever used external interfaces and mixers, so I can't really comment on your sound card question. But I can tell you from experience that while the mixer you linked is a nice one as a mixer, it's pretty lousy as a USB recording device. It only passes the main mix through USB, and NOT in stereo. None of the panning controls seemed to have any effect on the sound going to disk. It also has an annoying buzz whenever the USB is plugged in, though I imagine that's just an electronic problem in my particular unit.
In short, as a mixer and USB playback device, it's decent. For recording, be aware of its limitations and save your receipt.