Originally Posted by OWA
If the video codec is the same the movie will likely look the same on either format. That is, most reviewers don't notice a difference. If they're not using the same video codec. It's a more subjective. One might look a little sharper than the other but that might create other issues, for example. The audio specs aren't always equal though since a lot of the dual-format releases don't provide the same options, for some reason.
So, if everything is equal, pick which ever one you want.
What if one has HDi and the other has BD-J? Wait... that wouldn't happen, lol.