The DS and PSP can't really be compared. The thing is, they have very different price points - $169 vs. $129 - different control systems, different functionalities, different audiences and consequently different sales patterns.
I have both systems, and love both with a passion -- they're probably the two most satisfying gaming purchases I've ever made -- but they do very different things. The PSP is more of a high-end gaming and media device, with that glorious screen (which I've decorated liberally with assorted FF12 screenshots... sky-pirates ahoy!), while the DS is more of an intuitive, casual gaming device built around stylus input, which trades customization for ease of use.