I really liked the PS500 too and found it less fatiguing and with less pronounced mids than the RS1i. But I'd still go for the old bird!
I recently had the chance to compare my HP1000 to an RS1i and a PS500. The difference between the latter and the RS1i can be summarized as less mids, and a bit more treble and bass. I actually think it's less natural than the RS1i, but then it's a lot less fatiguing. I actually reviewed them in my website. It's in Portuguese, but I think Google Translate does a pretty good job! Bear in mind that I write for the Brazilian public, which contains mostly noobs, so I have to be pretty detailed. So it's quite long, but just jump to Sound and you'll be fine ;)