I chose Copenhagen because I think it is a harder challenge to take an existing city that is 'set in its ways' and evolve it into an eco-city. They set impressive targets and they have achieved so much. Of course it is very important that we are very careful when designing NEW cities but I think the first challenge is to take our existing cities and make these more sustainable. This is where the people already are and where change needs to happen NOW!
in opinion, Copenhagen is currently leading its way in the battle of Eco-cities because this city has passed all its developmental phases. Al the strategic measures are put in plan while Masdar has a long way to go. This city will be completed in 2020's so its not fair to compare such contrasting cities at one scale.
Copenhagen has been a steward for sustainable cities with urban labs to implementation of sustainability initiatives in the city involving the citizens are just some of the actions taken by the city. It can provide best practices for cities still on the journey of sustainability.