I guess I can comment on the audio.
I can really only comment on The Fifth Element, as it's the only disc I've seen enough to know the soundtrack well enough.
The DD track is slightly better than the old version. There's a bit more detail to it than old version had as the old DD track was compressed more than the Superbit version is.
I didn't care for the DTS mix on it though. You can argure all you want as to which is better DTS or DD. It's more about the mix, than it is about the compression scheme. I usually preffer a DTS track. Though sometimes the difference is non-existant. And sometimes the DD track is better, as I feel it is on the Superbit Fifth Element.
Though if you really want the best audio track, go with the LD release.