I like my iPod Touch, but there's no denying the iPod Touch is a poor man's gaming platform, with no buttons, relying completely on touch screen. It also has terrible stability; its OS is worse than anything Microsoft has ever released.
Both the DS and PSP have good games for them. I decided to get the DS because there's a ton of games that I wanted on it. It also has gone past the era of gimmicky games based entirely on touchscreen gimmicks and has good games for it being released regularly, unlike the Wii, which seems to have not got past that era yet, possibly due to much higher third party support of the DS.
Additionally, it seems there's a lot more games on the DS tailored for on the go gaming, like Professor Layton, Advance Wars, and Elite Beat Agents. In comparison, most of the best games on the PSP seem to me like they are emulating their console counterparts.