Aside from being burned by Sony because the slim's TV output only supports Progressive Scan/Widescreen support only for games, I have to say that the original is far better than the slim.
The original is sturdier, made better, less cheap. On the slim, the UMD slot/tray is a cheap piece of plastic with no eject button, with nothing much to keep you from accidentally fucking the insides when putting in your UMD. Also, the screen isn't protected much at all. The original has a cover of sorts over the whole face of the PSP, protecting the screen and the edges of the screen, the slim does not. The slim also has a smaller battery, with less life. Sony actually wants you to go out and spend more money on a better battery and battery cover.
The slim, however, is slightly faster with loading, and the browser is a bit smoother.
But I just don't trust the slim. It's not out of spite because of the TV output being crap, it's that it just feels so cheaply made. I returned mine. I was too afraid of it being damaged by accident.