Here are my personal opinions on this:
Bluray - I only buy Bluray of movies that have been recently released, like 2005 up. An exemption of this is the Star Wars movies, hopefully to be released soon. If I also buy older movies, then it will never end!
Dvds - Older movies that I like, with good audio and video, and really good supplements. Since DVDs are getting cheaper by the month, it's a good time to buy some of my older favorite movies on Dvd.
Download - Movies that don't need surround sound and high definition, like romantic comedies. And, movies that don't give any supplements worthwhile on Dvd or BD.