Since the movie industry is still blocking Apple from allowing re-downloads of movies into i-Tunes, I prefer to buy the disc and then rip my own DRM-free mp4. Either that or I buy the disc with the free digital download.
That way I automatically have the digital and the back-up copy and it usually doesn't cost more. I don't know, I'm probably breaking 312 DMCA laws by doing it, but it's just too hard to lose your digital media to computer crashes and changing technology. I get really tired of buying the same things over and over.
One word of warning, I'm started to see a lot of "Ultra-Violet" digital download offers with movies. Stay away from those.