JV did 30,000km of testing in the World Champion car before the season. At the time, running in practice was unlimited, and testing was too. The partnership between Williams and Renault was still very strong. Williams had stability, finance, and the best engine. Also, JV had won CART and the Indy500.
Rosberg isn't nearly as experienced, and comes into a team that is trying to recover from a couple of pretty awful years. Money is tight and they keep losing key staff.
So JV put the car on pole in his first race, taking full advantage of the 12-lap qualifying session. Rosberg made errors in his second quali run in Bahrain, but then so did a few others. And then Rosberg qualified 3rd on a track he'd never seen before in a car that has no real right to be there.
Time will be the ultimate arbiter of who is the better driver, but I think that Rosberg has done very much more with the tools at his disposal than did JV.