City policies are much more capable of maintaining the economy on a smaller level, as they know what their individual region needs. Too much national oversight only leads to bureaucracy, and a mismanagement of resources as the higher-ups misunderstand what factors into each individual region's microeconomy. City policies allow for more micromanagement of each region, to maximize economic output.
Very rarely do you see a local economy outlast or outdo a federal economy. Sure, there are certain cities that do better than the federal level--such as New York and Boston--but for the most part, federal law is the best way to go. Set the bar high federally and watch things improve.
Living in the city of Detroit all I can say is that they don't have much knowledge on how to help the economy and help the citizens of Detroit in general. So I would have to say that National policies would be way better for the economy. Detroit should follow what others have done, and improve on the economy that way. They should adept that the national policies.
Overall implementing economic beneficial national policies will eventually reach the local level and benefit everyone, I think local/city policies can tweak those national policies to make sure they are as effective as possible in the area but overall I think national policies are better at improving the overall nations economy.
I think that both city and national policies have an effect on a local economy, but ultimately, the national policy will be the policy that drives the success of a city the most. If it is national policy that jobs are permitted to be driven out to India and other places for a fraction of the cost, then there isn't much that a city policy can do about that.