Depends on who you talk to. Personally I like the 325i and the RS1i. Considering your a treble head like me I'm thinking either headphone would serve you well. Another route you might want to look into is the
Magnum Upgrade. The company who does this upgrade is a Canadian company and from what I've read they're quite good. Most Grado fans who have tried the upgrade seems to love what they're hearing. Right now the new PS500 seems to really be making Tribe Grado all happy and music crazy with what it can do. Probably the most sought after Grado, especially amongst the traditionalists, is the old school
Joe Grado HP1000's which go for a very pretty penny. The people who own Joe Grado's claim the HP1000 rocks better then any other Grado ever made. If you prefer to be rockin with one of a kind style and have extra cash to burn then I would go the full out crazed modder route. The best modder out there right now would be the dude who runs the site
Headphile. Personally I've always loved the pleasing aesthetics of the
Triple Point. A little warning Grado's are an acquired taste and people who like the Grado sound are infamous for being screwballs so be prepared to throw away all sanity for the sake of loving your music. Considering I'm one of those screwballs I think that's a very good thing. Give a Grado headphone a listen one day and see if you like it. There's always room for a new fan girl amongst Tribe Grado.
I haven't listened to either headphone but from all the impressions and reviews I've read the PS500 seems to be a rebadged slightly tweaked HF2. Read for yourself what
Mike from Headfonia has to say about the PS500. Next summer I plan on moving up the Grado higher archy and right now the PS500's are on my list of possibilities. I'll have to wait and see what happens but I've heard the RS1i and the 325i and can"t help but wonder what the PS500's might sound like. A lot of owners of the PS500 are saying the big news for the PS500 is it follows more in the Joe Grado smoother house sound vein and is a departure from the modern Grado house sound. Whether that's true or not depends on the ear of the listener.