HFCS and sugar are almost identical - both are about half fructose and half glucose. I don't think one is significantly worse than the other.
HFCS is a solution of unbonded fructose and glucose. The most common HFCS is 45% glucose / 55% fructose.
HFCS may be worse for you if fructose is worse for you. That said, the concentrations aren't that much different.
I'm not going to dig out the citations (because I'm not sure where I'd find them) but I'm pretty certain I read recently in a credible source:
* Fructose is worse for you
* ... but HFCS and sugar contain similar amounts
* HFCS is really bad for you
* ... but no worse for you than sugar (which is also really bad for you)