Ok, so I compared the two a bit.
HF-2's bass definitely has more impact. In general it's sound has way more body then RS1i. HF-2 gives a dark tonality to the complete sound spectrum though and could be considered somewhat laid-back for a Grado. (it's still not Sennheiser HD6X0 laid back offcourse but relatively in comparison with other Grado's).
The highs, or 'treble' if you will, goes further with RSi though. It's more upfront, more trebly, engaging and at higher volume it reaches the point of harshness and painful much sooner then HF-2. I also think RS1i is a tiny bit more detailed, or at least gives the impression of being more detailed.
It also has more of what I call 'dynamics'.
To be short: HF-2 has the 'body and slam', RS1i has the treble and mids texture.
For distortion gitars, RS1i is a bit more engaging and adventurous, but HF-2 is more complete because it has more body (but that comes from a D7000 lover, so I guess I can appreciate some bass and body).
Overal, I'd grab for RS1i with acoustical gitars and some lighter classic rock maybe. I'd grab for HF-2 with power rock, metal, progressive.
For other genres, I simply grab for other headphones, since they will do much better for my taste.