I agree that xfce is a natural choice for Debian. I suppose that most computers have DVD players, but DVD discs are more expensive than CDs. Even if the difference is rather small it feels unnecessary to waste a DVD when I CD could do the trick. Especially since you only use the installation disc a few times. Having a physical disc has a advantage over USB drives namely that it can be used as a rescue disc later on. Data on a USB drive tends to be overwritten and you might not always have another computer available.