Personally, I think Rosberg is somewhat underrated and Vettel is incredibly overrated.
Vettel is fast, with Rosberg I find it hard to tell. His cars have never been good enough to compete with the best and his team mates have never been good enough to compete with him. Nakajima was diabolical and Schumacher, as good as he once was, is a relative geriatric. But even with a decent car and good competition, I don't think he'll be one of the fastest on the grid. He's fast but not that fast.
But speed isn't everything, unless you're Vettel

Vettel is inconsistent, if this year has shown me anything then it's that. He's a hot head and he's immature and, as a result, he makes mistakes. Rosberg is proving himself to be the opposite, at least this season. He's had three podiums this year, so with a better car he could definitely be competing for race wins.
I think it's impossible to tell who is better than who when everyone has a different car and a different team mate. I'd say Vettel is more flashy and that usally engenders more hype and conversely, more derision. That doesn't mean he's a better talent.