Regarding Netflix vs Amazon. Netflix and Amazon both use 720p HD though Netflix is only in stereo while Amazon has 5.1 audio. Based on the bitrates below, Netflix has better video quality, while Amazon has better audio quality (though I don't think the NV1 supports 5.1 audio as other devices like TiVo or Roku do).
Available bandwith is the key though since both Netflix and Amazon will lower the video quality if the bandwidth isn't there.
If you are curious about Netflix's encoding, Netflix posted a lot of detailed information on their blog
about a year ago. Netflix has 3 SD encoding levels (375, 500, 1000, and 1500 kbps) and 2 HD encoding levels (2.6 mbps and 3.8 mbps). So if you want the highest HD level from Netflix you need to be able to have at least a 3.8 mbps sustained download speed (I was wrong about 3.5).
Amazon's doesn't detail their encoding specifics, but the individual shows list their encoded bitrates on their web site
for individual shows. Simply click on a show and then scroll down to the "Video Format Details" info. TV shows are streamed at 720p HD 5.1 audio with bitrates of 300 Kbps, 600 Kbps 480p, 900 Kbps 480p, 1.2 Mbps 480p or 2.5 Mbps 720p HD. Details about movies in HD aren't listed, but Amazon's help page says that they are also 2.5 Mbps 720p HD (though it says a 3.5 mbps connection or higher is required).