My CD has the former on the front and the latter on the spine. I've looked at many website discographies as well as the official Pink Floyd site and no one seems to agree. Regardless of the usual "the" issue in regards to tags and filenames and play-lists, which title did the band intend?

According to "The Complete Guide to the Music of Pink Floyd" by Andy Mabbett, original name was "The Dark Side Of The Moon", then Pink Floyd had to rename it to "Eclipse", because there already was such a disc.
Then they renamed it back to "The Dark Side Of The Moon" (which was also the title used on CD), but they dropped "the" shortly afterwards. So I believe the more accurate title is "Dark side..."

Note, that I'm not a "floydian". Just like their music