The PiP feature of 300 hasn't been replicated on Blu-ray yet. There was some talk of a double-dip coming in the future though.
There are a couple WB HD DVDs that are "better" than the BD counterpart in some way... Lady in the Water and Happy Feet (and previously Superman Returns) had TrueHD tracks on HD, but only DD on BD.
I have the Matrix and Harry Potter 1-5 sets in HD, but ended up getting the BDs later on as well when i found them for a good price.
I guess those of us that may have already bought in both formats could trade-in and sell the second BD copy... probably only end up with a couple dollars per disc though