The Memory Stick Pro Duo's are one in the same. (Just out of curiosity, where are you finding them cheaper?)

One note, however; as strange as this is, though the PSP plays mp4s, they will not play mp4s from a memory stick pro duo that comes directly out of your M1 camera... I've tried it, I have both a PSP and an M1. (The PSP will show off your M1 still photos, though.)

Hope this helps!
Susan