Quality is very connected to your bandwidth. NetFlix is a streaming service, so unless you have at least a 4 -5 mb connection you will not get HD. And the best sound you can hope for from Netflix is DD(?) 2.0.
Amazon, as it downloads, is not so dependent on your bandwidth - though a slow connection speed means a very long time for a movie. If you are at 5mb, that is about real time - I have a 3mb connection so I need to wait at least an hour to watch a movie being downloaded.
Picture quality from NetFlix - at its best - is 720p, and from Amazon you will be usually getting 1080i from your TiVo. Amazon also offers 5.1 sound on many titles.
So it is really a choice specific to the end user. We are NetFlix subs and usually stream at least a couple shows or a movie every week. We have downloaded a couple shows, in a one year period, from Amazon. So we use the NetFlix service weekly and Amazon a couple times A Year.
Price is the issue. At 3mb we get the best SD quality you can (all the bars lit up, but not the HD words) and that is good enough for most shows and some movies. If is is an action movie or other type where better sound and picture is desired, we have the Blu-Ray come in the mail from NetFlix. If we really like or want a specific movie, we just buy it.
Amazon charges you per download, but NetFlix is a flat fee per month. Do the math and see what works best for you and your situation.