There's also no reason not to keep my special edition DVD if I buy the BluRay.
Oh I would definitely keep the DE of TMP, since it so unique. It has some great extras that were made for it and of course the DE version of the movie that doesn't exist elsewhere.
I have kept my TMP DE for that reason.
As LOKAI of CHERON said, the Blu-ray presentation is the one to watch for the sheer resolution/detail and the awesome lossless audio track, it really lets Goldsmith's soundtrack breathe.