Quite funny that Ethan Winer in Lord Gaga's comment said that he had seen this discussion before.
I think it will come down to taste and needs.
I've been working with E-Mu, Motu, Focusrite and Apogee a lot and the pre amps differ in noise levels but I never really noticed any AD/DA differences.
Even when working with external signalflows like DA to compressors to AD works quite nicely with all of those machines.

When working with line levels only I wouldn't bother spending money on the soundcard.
You could always try the suggested test that Ethan talked about:
Take a cd player, record it via line in, load the recorded track from the cd into the same session and A/B, if it's "the same" sound quality, your good.

 

of course it does,not really during the mix but surely when u plug your instrument in,if u do.i had a presonus firebox and a digidesign mbox2 mini,same price range.well,the presonus was sold after a year,no comparison in the converters