I've skied the RC (in a 175cm, I believe) and I own a pair of 180cm iSpeeds. I'm 6'1", 195# and I ski primarily at Jackson Hole.
I think the two compare very, very favorably. I liked the RC a great deal and would probably own a pair if I weren't a rep for Head.
Comparing the two, I felt that the Fischer gives a tiny bit more feedback from the snow. That may be partially due to the fact that a 175 length in that ski is probably a bit short for my weight, but that's the pair I had the opportunity to ski. They felt "lively" and very easy to turn, but also still very stable at pretty high speeds.
The iSpeed has that classic, very damp "feel" that most of Head's sandwich skis have. Some people (like me) really love that feel and others think it makes for a ski that feels dead. I like it because the ski doesn't do anything unless I ask it to, and then it does everything I ask. It feels incredibly smooth, easy to turn, but just glued to the snow (in a good way) when your speed starts creeping up there.
I don't suppose that demoing is any sort of an option? If not, all I can tell you is that I liked the Fischer a whole lot. I like the Head even better, but I'm expected to. I honestly think you could flip a coin and be very happy with either ski.
Good luck with the choice.