Be aware that the compression quality of Netflix's SD material (the only versions the Wii will play) is noticeably worse than DVD quality. For stuff that isn't super visual, it's not a problem, but if you are watching a movie you want to see clearly, you're going to want to get the disc. Bitrates hover around 1.5 Mb/s, bursts up to 2.5 or so.
Their 720p "HD" streams not only have more resolution than the SD versions, but far less compression artifacts. They look noticeably crisper than DVD's, but still noticeably worse than Blu-Ray. You'd need another kind of playback device to play the "HD" versions, though. Bitrates hover around 3.5 Mb/s, bursts up to 6 or 7.
Regular DVD is usually around 8-9 Mb/s, with very little compression artifacts but not much resolution. Blu-Ray hovers around 25-35 Mb/s.