Not all smartphone users are created equal. Those who use iPhones are more apt to buy apps, a new report from AdMob says.

Roughly half of iPhone users purchase at least one app per month from Apple's (Nasdaq: AAPL) iTunes Store. Google's (Nasdaq: GOOG) Android and Palm's (Nasdaq: PALM) webOS didn't make out as well. Only 21% of Android users and 24% of webOS handset owners buy at least one app per month from their respective app stores, the report says. Research In Motion (Nasdaq: RIMM) was not included in this part of the report.

RIM's BlackBerry OS was responsible for just 5% of smartphone traffic in January, or one-tenth of Apple's share. Perhaps that's why Mike Lazaridis, RIM's co-CEO, has called for penalizing the iPhone. Palm's webOS accounted for 2% of global traffic, the same as Microsoft's (Nasdaq: MSFT) Windows Mobile.

No matter. Regardless of what Lazaridis says, data usage isn't the issue here. AT&T (NYSE: T) is perfectly happy to have the iPhone continue to be a data pig. Executives at the telco have said as much.

No, the big issue for its competitors is that the iPhone attracts bigger spenders. Combine that with its platform-sharing cousin, the iPod touch, which has 35% of users paying for an app per month, and you have a huge problem for Apple's rivals.

Developers, on the other hand, must love data like this. They're stuck with too many smartphone platforms to choose from. Knowing where the money is should make it easier for them to decide which OSes to write code for. (Platforms are only as good as the dollars they attract, after all.)

Put differently: If developers haven't created more software for Palm and RIM, it's because their mobile OSes -- good though they may be -- show no signs of being the moneymakers that the iPhone has already proven to be.

That's why I continue to short both stocks in my Motley Fool CAPS portfolio. Think I'm wrong? Kick up some dust in the comment box below.

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Fool contributor Tim Beyers is a member of the Rule Breakers stock-picking team. He had stock and options positions in Apple and a stock position in Google at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. The Motley Fool is also on Twitter as @TheMotleyFool. The Fool's disclosure policy is waving at you. See?