When I worked in commercial real estate, we used CB Richard Ellis market surveys as our baseline. You can get them free after registering at their slightly confusing website: http://www.cbre.com/USA/Research/Market+Reports/
As far as I can tell, downtown Chicago's market share is increasing relative to the suburbs; the same is the case in DC and probably NYC. The "jobless recovery" resulted in new towers downtown while vacancy rates remained stubbornly high in the suburbs.
