It is preference.
Have you heard any of the Prestige Series Grados? If you have, the PS500 is a bit different. The PS500 has a big mid-bass emphasis. It has a slightly thick sound because of this. Also, the mids are less forward than the Prestige Series models, but probably more realistic as well. There is still a treble peak so the treble is still there.
A lot of people love the PS500. I like it, but I don't love it. The bass is too emphasized in my opinion. It probably sounds great with certain genres, but I don't think the PS500 has very good genre bandwidth, even less so than typical Grados.
The RS1 is the best Grado I've heard. I've not heard any discontinued models or the GS1000 or the PS1000, but the RS1 has been the best of the bunch so far. I think the improvements you get from the Prestige Series to the RS1 are not worth the hike in price, but some people think it is worth the price jump. If the RS1 were a $400 headphone, I'd own it right now.