USB microphones are great for podcasters, and circumstances where our usual gain setting is not required. Obviously you can never add another microphone, so singing and playing guitar at the same time is impossible (for the main). The problem is really that most computers have terrible quality microphone inputs - so to get them costs around £50 plus for an interface where you can connect pretty well anything. USB mics that contain the interface internally and still cost cheap money are off course, less good. The cable doesn't;t do anything justice - they're meant to be totally transparent. Quality coming from the mic and the interface really. You're kind of in the middle. What is the purpose of the mic? What are you planning to do - that's more important. The D5 is a fine mic for the price, but it's a super-cardioid, so good at rejecting sound, including, if she hasn't got much mic technique yet, her! Not bad for PA's though and live sound.
The MacBook.
Oddly, the mic input on a MacBook is actually quite a bit better than the usual PC mic input, but the snag is that you presumably also want to hear what she's singing and the socket is already in use for the mic, so again - and interface can do both properly.