Ohh baby, my favorite topic.
-Fat has more speakers (4, not two, noticeably louder on fat, speakers on bottom of psp and lower part of front, not upper part of front)
-Fat weighs more, much bigger, very easy to tell
-Slim has video output to tv
-Slim is missing IR (infrared) port
-Slim WLAN switch is on top, not side
-Slim has less complicated disk slot (no metal bars, just empty)
-Slim has slide out memory stick slot, not door style
-Elevated buttons on slim
-More RAM (random access memory) on slim allowing for faster loading times and option to cache umds resulting in more battery life
-Supposedly more efficient battery usage
-Charging through usb mode
I think the slim is better, but if you already have a psp, don't get it. Guys there are a few flaws. Its not completely better than the fat. Basically the same thing. It's better, but not really worth an upgrade. And one other thing I noticed was that the control stick feels a little cheaper and plastic-like whereas the psp fat's feels very sturdy. Kinda took a lot out of the plastic to make it weigh less, I guess. Hope this helps.
got em both