My opinion is based solely on a perception that marzocchi forks are better than rockshox darts. Basically think of rockshox as a very wide range - from very basic, entry level - to very very good.
i.e. the worst are really basic, entry level forks, but the best are professional standard, e.g. boxxer. Marzocchi and fox are in-between, more of a narrower band.
However that's not to say those particular forks are a pile of shoot - they're probably perfectly adequate.