THX is basically a set of guidlines so movies don't sound so bright in ones home. They also have a set of guideline for theaters to follow as well, among a few other things like the THX crossover.

DD compresses their signal to about 400kbps.
DTS compresses their signal to about 1.5 Mbps.

DD claims there compression format is more effecient than DTS therefore the bps is not so relavant.

Arguments can be made for both sides, I think DTS does sound a bit better in most of theater I have worked on.

I was not aware of that DTS was bumbed up 4db, interesting...