HDMI is a digital signal, component is analog. Whether you can see the difference depends on the quality of the content and the receiver. Properly driven and terminated analog signals can provide just as good a signal quality as digital but the extra conversion steps required can introduce signal degradation. You probably won't be able to see any difference in the over compressed content coming out of a so called HD digital cable box.

from http://www.bluejeanscable.com/articles/dvihdmicomponent.htm
"So, which is better, HDMI or component? The answer--unsatisfying, perhaps, but true--is that it depends. It depends upon your source and display devices, and there's no good way, in principle, to say in advance whether the digital or the analog connection will render a better picture. You may even find, say, that your DVD player looks better through its HDMI output, while your satellite or cable box looks better through its component output, on the same display. In this case, there's no real substitute for simply plugging it in and giving it a try both ways."
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