For me,PS-1000 do everything the 500 does and does it better,the bass is a bit deeper and better controlled,the mids sound very natural,as opposed to the very slightly chesty sounding mids of the 500,and to be honnest,some might even prefer the sound of voices through the 500,as it gives them a little heft and warmth,the 1000 highs are definately more extented with a little more air,and finally the 1000's soundstage is wider and deeper.
As far as comfort goes,the 1000 are very heavy,i have a small head,and i like to listen to music with my feet up and my head slightly tilted back,it took a while,but i finally found a way to ajust them so that they woudn't fall off.The 500 are much lighter,altough i had and even harder time trying to ajust them,it's like they don't have much clamping force,i had the RS1i at the same time to compare,and found them to be very comfortable,but remember i weight 132 pounds and have a very small head,so this shouldn't be a problem for most peoples,by the way i also have small ''Grado ears'' so the L cush are no problem for me either.
In the end,it's all about the music,and,in the two weeks i've spent with the 500,i remember saying to my girlfriend that i'd never got ''lost'' in the music as i did with these headphones,i'm still trying to figure out what it was that made me feel like that,the only explanation i can come up with,is the fact that they sound very warm,with good bass extension,lush mids,and politely detailed highs,they never sound offensive,they're very easy to like.