AdMob (http://metrics.admob.com) — one of the world’s largest mobile advertising networks — has compared unique devices running the Android and iPhone operating systems (OS) in its network in the April 2010 “AdMob Mobile Metrics Report.”

The Android OS had its greatest concentration of unique devices in North America (75%) in April 2010, followed by Asia (12%) and Western Europe (11%). iPhone OS devices are more broadly distributed worldwide, but also had its greatest concentration in North America (49%), followed by Western Europe (28%) and Asia (14%).

In the US there was a 2-to-1 ratio of iPhone OS devices to Android OS devices; worldwide, that ratio expanded to 3.5-to-1. In AdMob’s network there were 8.7 million unique Android OS devices and 10.7 million unique iPhones in the U.S. in April 2010. But when non-phone devices running the iPhone OS — the iPod touch and iPad –– are included, the number of unique devices jumps to 18.3 million in the US. Worldwide, there were 11.6 million unique Android OS devices, 27.4 million unique iPhones and 40.8 million unique iPhone OS devices.

The top five countries with the most unique iPhone OS devices were the U.S (44%), United Kingdom (9%), France (6%), Canada (5%), and Japan (4%). Over the past 12 months, unique iPhone OS device growth was strongest in Asia (474%), followed by Oceania (367%) and Western Europe (269%).

The top five countries with the most unique Android OS devices were the U.S. (75%), China (8%), United Kingdom (3%), France (2%), and Germany (2%). There were more Android devices than iPhones in China in April 2010 in the AdMob network.

AdMob stores and analyzes handset and operator data from every ad request in its network of more than 23,000 mobile web sites and iPhone, Android, webOS, and Flash Lite applications to optimize ad serving. Each month, the AdMob Mobile Metrics Report aggregates this data to provide insights into major trends in the mobile ecosystem. AdMob defines a unique user as an individual device that has sent at least one ad request to the AdMob network in a given month.