Based on IGN scores over the past 4 months the PSP not only has more higher rated games, but PSP games are more consistant in terms of quality. The majority of its game hover from mediocrity to excellent, while DS games are all over the place, including a recent game getting as low as a 2.0 out of 10.
In the past four months, the PSP has had 11 games rated in the 8.0 range, and 3 rated in the 9.0 range. The DS has had 6 games rated in 8.0 range and none rated in the 9.0 range.
Even the lineup for future releases shows PSP games at an early lead. Grand Theft Auto: Vice City Stories will be a huge seller and a definite critical success, as well as Lumines II, and SOCOM Navy Seals: Fireteam Bravo II.
On the DS side, Fullmetal Alchemist: Dual Sympathy is shaping up pretty well, but other than that, a lot of licensed products are being released (i.e. That's so raven, Biker Mice from Mars, Over the hedge, etc.). A few classic games are going to be released including Final Fantasy III, but the PSP is going to have an Official PS1 Emulator, which will allow PSP users to download PS1 games via wifi.
So I don't know. I think the DS has the MOST games releasing every month, but as for them being better, I'm not really seeing it. I suspect that the new DS lite is pushing sales as well as Mario, but as we get to the Christmas season, I see the PSP gaining a lot of steam. Especially with the reprice of UMD's, release of the 3.0 firmware, PS3 connectivity, the release of the PSP Camera and GPS (which is already sold out on Japan's Amazon.com), and adverts that highlight the PSP's multimedia capabilities.