I think that in the end, there's one simple way to determine which device you want; If you like racing and fighting games a lot, go for sony/microsoft. If you want more adventure-like games, innovative ideas build into games or anything else that is somewhat special, go for nintendo. It's always been like that and probably always will be.
Besides, most features that they add to the PSP are things we already could do, on a PPC. Those are not really arguments (to me at least) to grab the PSP over the DS.
Now I wonder when sony will starting making 'special' PSP's, like what you can do with the DS... Besides the iPSPod of course ^^.