Which DAC is better?

Check out the PS Audio PerfectWave DAC MKII.

Can comment only on the qb-9. Its a really good dac on its price range, very musical.
If you want to spend maybe less, the DA-160 is a nice option.

Try a Tranquility DAC

I think all 3 choices are very good. Ayre happens to be my favourite in solid state electronics. Naturally, that would be my first choice. I've also had Arcam CD players. Both products sound great, but they are very different from each other. Arcam digital products are very listenable. They have a refined, polite sound that you can listen to all day. The Ayre is completely different. The sound is very liquid and pure. It has tons of resolution, as well. Some say it is analytical but I would disagree with that. For all its resolution, it is a very musical component. Given the vast difference in sound between them, I would strongly urge you to listen to both before making a decision. I am not that familiar with the Sim but if I had to guess, it would fall somewhere between the other 2.

I own the Simaudio 100D, the little brother of the 300D, and it's a wonderful, natural sounding DAC.

If your primary source will be USB, though, the 300D is probably not the best choice. Simaudio makes it very clear in their description of both the 100D and the 300D that they put their effort into the SPDIF circuits and include the USB input mostly as a convenience.