Personally I dont think they are comparable.
And I agree with whoever said about the games selection; tho I do agree that the PSP is so much better and can do more stuff, its being a bit slow on the games front...
Once again it all comes down to the user, psp offers many more features, but most of Joe public people wouldnt know how to use them effectively. Like putting free things on your psp, or resizing movies to place on psp, or surfing the web on psp. (and homebrew only works on v1.50 atm, and Sony is enticing people to update with more desirable options to rid of it)
Did it cross your mind that DS is actually appealing to a different audience? IMO, as per usual Nintendo is aiming at a younger generation (who most likely could not afford their own purchase) so cheaper prices make it a much better option to keep the youngun' happy.
The PSP on the other hand, generally seems to be aimed at a slightly older audience (look at the launch games), and with its extra features; a more technically savvy audience.... who can afford to blow some money on a more impressive, expensive gadget.
Most likely DS has outsold PSP, but probably due to its cheaper pricing.