Good analog mic + good soundcard = good sound
Good USB mic = good sound

“Digital to digital” does not exist as all audio recording goes from an analogue to digital conversion, in a chip known as an ADC (analogue-digital-converter). The only difference, is where this takes place. In the unit, or in your soundcard. Put simply, all sound is analogue in it’s nature and must be ‘sampled’ into a digital version.

Now, a cheap soundcard (particularly in laptops) is going to sound pants, whereas a proper soundcard is going to sound good. The “white noise” is actually quantitization error for the most part, and digital interference in the minor (filtering digital noise from electrical equiptment is fairly easy, if it were hard highly sensitive units, say your CPU, wouldn’t function). Moving the ADC external does little to counter this, as digital noise is generally caused by how “clean” the electrical signal is, something that USB power suffers in equal measure to motherboard power.

What makes the difference? Well generally the ADC in external mics is better than the ADC in internal soundcards – recording is actually a seldom used function. Swap this for a decent soundcard and you will reverse the trend. Also, USB mics are filtered to only respond in the vocal range, cutting out most of the white noise. Try to pick up some music with a high dynamic range through a USB mic and the unit usually responds poorly.

Finally, there are many situations where taking an analogue signal is very beneficial, especially if you are looking to gain the signal.

As ever, generalizations of what is best are not recommended and the best one depends on application. If you are looking just to record voice / Skype on an average laptop then USB might be the best – for predictable quality and digitisation you can control, analogue.