This is sort of a silly comparison because the DS and PSP have been around for around 5 years and the iPad hasn't even been around for 1. The newest model of iPod touch came out a few months ago. The odds are very good that people who like games that own either device would have already owned a DS or PSP.
In terms of game sales the $0.99 price point makes a huge difference since it means you can buy around 30 iPhone games for the price of 1 current gen handheld title. Some games like Crimson Gem Saga and Phoenix Wright are even released for both handhelds and iPhones.
But it's not solely a gaming device, and the comparison makes no more sense than laptop sales vs Xbox 360 sales because they both play games and media.
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