As you've seen, the two cams are extremely similar. They use the same CMOS sensor and both record via HDV. In addition to the differences pointed out previously:
- the HV20 also has an HDMI out, while the HV10 does not, and
- ignoring ergonomics, the HV10 is smaller and lighter, which is a good thing.
I'll be buying my HV20 soon, so other than the quality time I have spent with it at Best Buy, I don't have any direct knowledge. However, from all of the reviews I've read (and I've read just about everything out there), the low light performance of the HV20 is worth the price difference. I'm just a casual user who will be recording the family, our travels, etc. A fair amount of that recording will be indoors with less than great light, so the HV20 makes sense. My wife agrees that, for our anticipated usage, its worth $200 or so to get better low light performance.