I have owned the RS-2's/1's and the 225's/325's/325i's...
no need to repeat what others have said..
just wanted to add that if you mean to select from the Reference line between the Prestige line, I would opt for the RS-1 hands down.
Only, try and audition a pair of RS-2's first...as they (to my ears) bear a closer likeness to the 325's than their bigger bro the RS-1..
The RS-1 and the SR-225 have a much closer sig sonically than do the RS-2's VS the RS-1's..
Actually, the SR-225 is probably the closest sounding can to the RS-1 IMO..and is the best value overall.
The refined sound and sparkling highs, as well as the increased headstaging, are only part of the reason I rec the RS-1...The biggest factor for me and for some is the materials utilized in the production of the RS-1 Headphone..the inter related components and cup holders are made of metal instead of plastic, and there are basically more quality parts used in the RS-1 which is nice.
I have never heard the MS2i's but own the MS-1's and used to own the MSPro's, and can tell you from those two sets, that they are sweeet. More of a neutral presentation, with much less emphasis to tibre and midrange on the Aless phones...but just as fast, responsive, and capable with Rock...
The cymbals/percussion on the metal grados contain much realism, but not as balanced in terms of the rest of the song, and its respective volume, meaning that the sounds in the lower register are there, but leaves my head ringing more than the plastic or wood Grados do..
The metal Grados are all awesome to my ears/eyes, especially the older Joe cans... but the 325's/325i's are a bit much in the upper registers for my taste...plus, the fatigue factor and comfort level of the 325's/i's, and any of the metal Grado monsters really, are something of an acquired taste...as they weigh heavy on the dome and need to be "adjusted" more on the head, maybe not a caveat for some.
Conclusion: the price for the RS-1's when you are looking is down to $550 or close for brand new, used for less..so is already around $150 cheaper than they used to be, something to factor in maybe.