Here's something for you. The PSP has sold roughly 41 million units worldwide, according to Wikipedia in August. Roughly the same time (Spetember), the DS has sold 84 million.
The highest selling game for PSP is Monster Hunter, sold 2.55 million units. The highest selling game for DS is New Super Mario Bros. (Nintendogs has multiple versions and doesn't count), which sold roughly 15. Including games like Nintendogs and Pokemon, the DS has seventeen games that have sold more then Monster Hunter. Say what you will about the DS, but the sales speak for themselves.
Agreed. The PSP has had limited success in outdoing the DS, and the sales seem to back that up.