I stumbled upon this thread, and how fortunate as I myself am in the process of deciding between these two exact headphones. I'm torn...
I listen to roughly 40% Rock / Alterative Rock, 40% Trance / House, and 20% Metal / Metalcore. So as you can surmise, I naturally love the Grado sound. I've heard all Grados up to the RS1, except the RS2. The RS1 I liked a lot, but couldn't justify the price. So lately I've been thinking of trying the RS2, and if I like it I'll look into a used pair.
Obviously Grados sound great with Rock and Rock related genres. But considering the other half of my music selection, EDM; from what I've read the HE-400 will handily out-perform the Grados at this task. So I ordered a pair of HE-400 a couple weeks ago, which will actually arrive tomorrow. I'll be able to compare them to my pair of SR60i.
All that said, I really do enjoy Grados with Trance. But I'm curious if that's simply because I've not heard a pair of open headphones with deep bass response yet (we'll find out tomorrow!). I've heard some closed-back phones (M50, DT770, K550), but was too turned off by the closed-back sound to enjoy the bass response; Trance needs an open headphone, as it needs to sound airy and ethereal...