Mobile ad network Millennial Media is out with its latest monthly Mobile Mix report, showing that Android impressions have caught up with Apple’s iOS in the U.S.

Within the 58 percent of devices that are smartphones, both Android and iOS hold equivalent 38 percent shares. The 58 percent smartphone penetration is higher than the broader market because it includes Millennial’s display ad network only (skews to higher-end devices).

Looking at these monthly reports over the past year shows a steady rise in Android impressions, as Google’s open model allows it to scale across various carriers and devices. This contrasts Apple’s vertical integration for iOS (software and hardware built under one roof).

For Apple, this has quality advantages but quantity disadvantages. Google is the opposite — one of the many ways the two companies  differ (see our recent report). We predict, as many others do, that Android will hold the second-largest global device share behind Symbian by end of next year.