Realistically, for most of the movie it would look indistinguishable. Certain scenes, however, would looks a bit softer on HD DVD bit-rate VC-1 (from what I've seen of that codec "at it's worst"). Others would show just a smidge more artifacts.
Would you notice a difference if you didn't know which version you were watching and weren't scrutinizing? Probably not. Would you be able to tell the difference in a side by side comparison. Yes, but only during more complex/difficult scenes (night, motion, etc.)
While I'm willing to believe that the extra bitrate doesn't matter all that much in the end, it still matters. I'm NOT willing to believe that VC-1 is made with magic pixie dust whereby it looks equivalent to all other encodes at any bitrate. There's just too much coincidence there: the max bitrate of HD DVD just happens to be the point where a particular codec achieves "perfection" which just happens to match the maximum size of the discs? You couldn't engineer that tight if you tried.
It's time to face logical facts: *if you are being nitpicky* AVC and VC-1 can look better on Blu-ray than HD DVD due to the higher bitrate. However, if you are that nitpicky you probably don't enjoy much of life anyway, so you won't be happy either way.