I see both sides (I am ICU and have worked the floor and ER). The argument for more pay, to me, is that so much more is expected out of us (at least where I am). Knowledge, leadership, expertise, floating all over, etc. So I don't really see it as an issue of who "works harder" - clearly the floor nurses usually get that honor. To me, its the constant vigilance, etc, that make the ICU so exhausting. I don't really expect any more pay though, simply wouldn't fly.
I am CERTAINLY not saying that floor nurses don't have knowledge, etc. But there is a drastic difference in the two work areas, in my experience.
On the other hand, critical care experience can be worth its weight in gold and its own reward. I believe a critical nurse is much more marketable, and one that can work in several areas even more so. Get CC experience, and learn to float joyfully (or at least with a decent attitude) and you'll be welcome a lot of places.