Rob is just as good technique wise, without a doubt. I believe he could potentially write some stuff that could really push metallica forward, like his skills on spanish guitar are incredible, and he has some powerful funk on bass too. The only thing is, I would imagine he feels very overwhelmed with Metallica, specifically James and Lars. No musician will ever crack what those two have developed since Cliffs death, no matter how good they are.
Yesterday don't mean SHIT.