Every angle of the dvd/blu ray argument has been hit, my opinion goes along with what most have said already.
Depends on the movie (Evil Dead isn't a must buy on blu ray if I already own the ultimate 3 disc edition),
if I already own it
prices, and etc
One thing that doesn't get mentioned is the packaging, most blu rays come in smaller cases, packages which yes saves on shelf space but sometimes it's kind of sad. Point in case Mad Men, I have seasons 1-4 of the show and they whoever oversees the packaging making really do a good job on the fold out case, layout, I like it I'm a sucker for that stuff if the film or show deserves the goods. Now blu ray is on my shopping list this end of the year and I'd like to get season5 on blu ray but damn it's a skinny case, nothing big or grand about it.
Just a personal thing.