FORTUNE — The early preference for the iPhone 5S over the iPhone 5C has faded a bit in the week since their release, according to a report issued Friday by Localytics, a Boston-based mobile analytics firm.

As of Sept. 22 Apple (AAPL) was selling 3.4 units of the iPhone 5S for every 5C. By Sept. 26 that ratio had contracted to 2.9 to 1.

Localytics also found regional differences in the 12 cities it studied.  San Francisco — a hotbed of app developers and other early adopters — showed the strongest preference for the more advanced 5S (85% vs. 15%).

Philadelphia, at 72%, showed the least. The company did not offer a reason why.

Below: The 12 cities ranked by percentage of active iPhones. In general, the bigger the city, the more popular the new models. The exception is San Francisco, which came in third even though it is the 11th largest market.