On Sept. 1, Steve Jobs announced that there were more than 250,000 apps to choose from in Apple’s (AAPL) App Store.

On Saturday, two websites — VentureBeat and MacNN, it’s not clear which was first — reported that the count had crossed the 300,000 mark.

Fifty thousand new apps in the space of six weeks? That would be unprecedented — if it were true.

It turns out, VentureBeat and MacNN have made a rookie mistake. They’re using a number from Mobclix, an ad exchange network that knows on which apps its ads have appeared, but not whether those apps are still available for download.

A better way to count iOS apps is to write a program that runs through the App Store looking at all the titles and comparing that list to the titles that were there the day before. That way you know which apps are still live and which, for whatever reason, have been pulled from the store.

That’s the method used by 148apps.biz and AppShopper.com, our two favorite app trackers. Even their numbers aren’t perfect. For one thing, neither has found an efficient way to count apps that are available in Apple’s overseas App Stores but not in the U.S.

According to 148apps and AppShopper, the number of iOS apps available for download in the U.S. as of Sunday morning was just shy of 280,000. The breakdown:

. . .

Google’s (GOOG) Android Market Place, the App Store’s nearest competitor, is catching up fast. As of Sunday morning, 113,123 apps had appeared in the Market, according to AndroLib.com. It’s not clear how many of them are still active, however. As reader Mark in Boston points out, Google mentioned in its earnings report last week that there were 90,000 Android apps in the Market Place.

See also:

• Apple App Store: 250,000 and counting
• 11,700,000,000 song downloads and other Apple milestones
• Why Apple purged 6,000 sexy apps

[Follow Philip Elmer-DeWitt on Twitter @philiped]