I don't know if the LCD framebuffer on the iPhone is 16, 18 or 24 bits -- what I know is that by default, the Apple provided OpenGL skeleton will render to a 32 bpp frambuffer and dither.

I believe most serious games correct this (and set it to the more appropriate rgb565, like what we have on the N900). Easy way to solve this question is asking the game developer , but my vote is that the iPhone game also uses 16bpp and thus the difference in color quality is either a bug or a caused by something else.

As for speed, well, the SGX on the iPhone (3GS) is just better and also has way less pixels to move.