...the 3DS is better than the PSP as far as graphics are concerned - the two systems are a generation apart.
To be more precise, the 3DS has a far stronger CPU (An ARM11-based dual core working at the frequency of 268Mhz compared to the 1-333Mhz scalable MIPS) and GPU (the PICA200 working at 400Mhz with 4MB on-board VRAM of the 3DS compared to the 166Mhz Sony's custom chip with 2MB on-board VRAM) and it works at a higher resolution (the PSP's resolution is 480x272, the 3DS's is 800×240 pixels, 400×240 per-eye in 3D mode). To top it all off, the PSP has 32-64MB RAM depending on the model, wheras the 3DS has 128MB, plus it's in a faster standard.
All in all, the 3DS beats the PSP in every aspect, plus it has glass-less 3D, so if you're in for the graphics, you'd want a 3DS. That said, there's an even beefier console out there - the Vita. Have you considered that?
I won't really link you to any site other than Wikipedia - articles on both consoles, their specs as well as articles about the GPU's and CPU's are freely available.
As an additional resource, I'll post a graphics capabilities chart that compares the DS, the 3DS and the PSP - courtesy of IGN.com.
As you can see, the 3DS beats the PSP in every way imaginable apart from the Vertex Performance which couldn't be properly measured at the time of creating the article.
EDIT: Corrected an error, kudos heartgold.