Sound cards and DACs serve different purposes.
Sound cards are meant to take the processing load off of the processor itself, thus giving better performance in games and such. The downside is that they take up a PCI slot, which many people would prefer to use three-way SLI or something else. They also offer various software and enhancements that make gaming and movies more enjoyable, but that's not to say they're horrible for music listening when these enhancements are turned off.
DACs are meant for heavy music listening and as a sound card alternative, and often times for laptop use where PCI cards aren't an option. I bought the uDAC to use with my M17x laptop but was wondering if it would have been a good replacement for my desktop sound card. It's not horrible, but I don't think it's worth it.