66 million versus 30 million speaks for itself really. I would say DS because like Wii, it offers a unique interface which affords new gaming possibilities and experiences. It caters for a broad demographic with its more diverse range of titles for those with either hardcore or casual persuasions.
The DS appeals to me in particular because I am more interested in its gameplay functions rather than the added multimedia capabilities of the PSP, which I regard as peripheral. If such functionality is important to you however, then I guess the PSP will always be superior in that regard.