Its difficult to do a comparison. It's not like you can turn one off and do a compare with the same hardware. I have a dual dac that i like and a single dac. It all really depends on the implemntation and voicing.
Just look for a DAC that matches your taste and forget about the rest.
Like Bru said, most DACs will sound different so whatever you like best will be best for you. I've not tried it myself, but my friend tried the AK100 and AK120 side by side. This is an example of almost identical hardware besides the fact that the 100 uses a single DAC while the 120 uses Dual DAC. He told me the main difference was in the soundstage where the Dual DAC produced a much better soundstage all around.