For me it doesn't matter if someone is winning or only scoring points to see which one of them is true star of the future and potential world champion. Nor does matter if someone just won in inferior car, because WDC challenge requires much broader set of assets.
Looking at Maldonado's and Rosberg's season, irrespective of their wins in superior or inferior machines, I would say Maldonado is more likely to challenge for title. There is one, quite huge obstacle on the way - no mistakes allowed - which I would guess will be a quite a problem to overcome, but the problem for Rosberg is much bigger - his consistency. He's very much like Button, great on his days, but average on others and there is no solution for that, as that is part of their talent.
Looking at current grid I would say Perez, Grosjean, Maldonado and Hulkenberg could be world champions in the future, but they have to a) be tested against good benchmarks and b) eradicate their mistakes. I see no light in the tunnel for Rosberg and di Resta, as their problem is consistency. In 2013 we should see in which group Toro Rosso boys, Bottas and Gutierrez are. Also we certainly will see if Perez is up to the challenge and what kind of response Rosberg has to Hamilton. I think Perez can pass this exam, but Rosberg won't. In Mercedes there won't be any of this 'McLaren equality' sillyness, they hired Hamilton to win and they will focus on him and his wishes. It doesn't mean Rosberg will be sabotaged or something like that, but it will be up to him to show he can match or even beat Hamilton, pretty much the same situation as in Ferrari, where Massa showed in the last few races Alonso is not unbeatable, despite being clear number 1 in the team. Exciting stuff ahead.