Honestly, either one would work. It really comes down to your budget. If you don't have the ability to buy a DAC in the near future, I'd recommend the Xonar first. It has a wonderful and powerful headphone amplifier built into it, and fairly solid DAC built into it, and a higher quality digital output (if you want to get a higher quality DAC later, you'll appreciate this). There's a lot of reviews around here on it.
Understanding a DAC can be a little confusing at first, and I think you may be a little confused. Your music is stored in a digital format. To listen, it is converted to an analog signal (Digital to Analog Converter). This can happen in large boxes, or as simple as a single chip in a larger device. This step is rather important. If the DAC produces a good sound, the amplifier enhances that sound. If the DAC produces a mediocre sound, the amp enhances that, but it will not fix the low quality. For example, if the DAC produces a hiss or a hum, the amplifier will produce that hiss and hum as well.