yeah, there's literally no difference that's worth noting in a fat and a slim, i've used both, the slim is a little bit lighter, but come on, we're freaking MEN, we don't have girly hands ..
other than that the extra ram is completely useless since we're running games off of the mem stick anyway (i-e no umd read times at all), unless you are interested in using Skype on your PSP (lol) there's no reason to buy a slim.
oh one thing though, the slim can do TV out, but playing PSP on TV would kind of kill the 'portable' aspect, so it's your call.