Not stupid. The High Def cameras that I am familiar with,
will downsample the video so that the output is Standard Def. That means you can shoot in high def (to be used in the future), but still work with standard video for the present.
Why the HV10 rather than the HV20? The HV20 is an improved camera. I haven't used it, but I have seen mostly positive comments. Some folks have complained about small issues like how tightly the battery fits.
I don't know anything about the other models you mention.