No contest. Not RS1i. RS1. I'm sure the RS1i's are good too, but I went for the older model. I believe it has a slightly wider frequency range and different impedance.
One note, they are open-backed headphones, so you cannot track with them. But if you are mixing, you probably do want to be using open-backed headphones.
I had this same question a couple years ago and did quite a bit of research, including comparing several different models/brands in person. The Grados by far reproduce the most detail, and are pretty flat in their frequency response. I actually mixed my own album almost entirely on these headphones in my apartment. Of course, I tested my mixes on a few different sets of studio monitors and cheaper headphones, but I really didn't have to change anything. Seems like if I get my mixes sounding great with these headphones, it almost always sounds perfect on the monitors. Bass is always one thing you'll have to watch out for, though, when mixing with headphones. And sure, it's not best to mix on headphones, but sometimes you have to, and you should definitely always check your mixes on headphones, earbuds, car stereos, ipod docks, laptop speakers, etc... BTW, I have a second pair of headphones that I use A LOT that I recently saw at Urban Outfitters for $30. No joke, they sound great. Whatever you do, just watch your volume level.