Van Burren on the DS would have my vote. Imagine a proper portable fallout game, plays like the first 2 with the van B storyline. I think it'd work, look at the best DS games they're mostly turn based and/or feature loadsa dialogue with NPCS.
"the psp doesn't have analog" "the DS has something like a mouse" That is more than enough reason for it to be for ds over the psp if it's on psp it will be an fps fallout game no questions asked, if it's on the DS it will be more like a real fallout game in the since, it could even be a port of fallout 1.
Hmm.. I dont know about the new versions of the PSP but i remember the old ones didn't have a analog, or if the PSP does have analog the DS still has the biggest advantage anyways, it has touch screen which is somewhat like a mouse.
i would prefer psp version. though playing on ds has some perks, psp is stronger system and i believe gameplay cutbacks wouldn't be necessary (it would allow "bigger" game)