Second this. The videos that itunes provides for download are often of lower quality than what is available for streaming (720p vs. 1080p). I haven't done a direct comparison between Vudu streaming vs. Itunes downloads, but I'd assume that it is of higher quality than the downrezed itunes downloads.An alternative, legal, way... sign up for a Movies Anywhere account, link it to both iTunes and Vudu, use Vidu app on TiVo to watch movies. (also works with Amazon instead of Vudu)
Also, Movies Anywhere is generally a great service which I am amazed actually exists, given virtually all of my past experiences with how these companies treat digital video.