Tough love answer: It depends, and there is no way to know for sure.
In theory, HDMI is supposed to be the best possible connection for HD material. But, it is still 1080i or 720p that is being carried. So, in practice it is often found that HDMI does not produce a better image, but the exact same image, or sometimes an inferior image when compared to component.
I won't switch to anything different than component video for a while yet. At least until more A/V receivers are available with HDMI switching built in and 1080p support.