I mostly buy DVDs.
Blu Rays are almost always of better Sound and Picture Quality, even with low budget and old movies, but:
a) I usually am fine with the quality of an upscaled DVD
2) I'm the sort of guy who lets friends borrow DVDs, if not outright forcing them to borrow films i find awesome, and not very many of my friends have, or need, BR-Players.
III) DVDs are cheaper, and i buy a lot of movies.
However, with Blu Rays becoming cheaper and more movies getting DVD/BR Combos, i'm starting to by more "blu" movies.