All of them have their advantages but it really depends on your viewing pleasure.
For some Blu-ray is the only way to go. The picture quality is in Hi-Def and is just beautiful but that is not to say the DVD is not the same, in fact you really have to see the two side by side to see the difference.
Others want to have the advantage of being able to have access on their phone, tablet or laptop so Video on Demand is the choice for them. Again, remember that you need to have access to an INTERNET connection to view VOD.
What we have seen some of our customers do is purchase both VOD and a DVD/Blu-ray. They love the flexibility of being able to sit in the whelping box or watch the instructions on how to teach manding.