i don't like slim psp's they don't have IR port (using your psp as a remote control or ps2 controller)
the feature of external screen ain't any good
It does not handle 1.5 firmware (or 1.5fw plugin for custom firmware), so there is loads of homebrew you cant run
it has less battery capacity
i can only guess its harder to do some of the cool mods on it, since its even smaller and more compact.
well other than that i guess a slim is as good as a phat one