A lot of people actually class solar panels as green roofing. I guess the word green can either be literal or metaphorical. France's new laws for example permit solar panels as a green roof.
In terms of your question though, I think it's a matter of individual gain vs collective gain. Solar panels might benefit you and your wallet more in the long term, but real green roofs - if widespread - open up the possibility of large scale in-city crop growing, and would do wonders for a city's positive oxygen and negative CO2 levels.