tried the slim. traded it for a phat and a 4GB memory card and tekken DR. didn't like the build quality of the slim, felt plasticky... and had a rubbish UMD door.
buttons feel the same as the phat - same rubbish analogue thing
the phat feels much heavier, i've dropped it a few times and nothing's happened. much better build quality, just the battery door doesn't fit on totally, it feels like it's digging into my fingers.
the slim was good, but too light and too papery...
the phat was too heavy, but felt much more solid. i'm buying a psp slim, though, because i'm thinking for the future. the phat can strain my handsafter a while because of its weight and i think that's the deal breaker here. plus my phat has some dead pixels - not stuck.