The masters series will be hard to win, due to Nadal being around. He could definitely win the smaller tournaments (Munich, Buenoas Aires, Acapulco, Gstaad etc.) regularly. I think he could win a few masters titles on the surface before he retires though. And, as part of my prediction for his domination in 2013, I see him winning Roland Garros with amazing claycourt play. The guy is definitely not a clown, and at his best on the surface, is top 5 at least.
Yes, masters series will be hard to win for Murray but not due to Nadal...there are plenty clay courters who could have Murray for breakfast. Check out Murray's masters history on clay and you'll see that Nadal is one of MANY he has to worry about. Just on top of my head, aside from Nadal...Federer, Del Potro, Djokovic, Ferrer, Soderling, Berdych, Wawrinka, Verdasco, are all superior clay court players to Murray. Bellucci is another one who will consistently give him run for his money.