The DVD/Video in the parks shows more of the rides than the promotional video does. As previously noted, it does not touch on other parts of WDW, only the parks.
The park video takes more of a "low key" approach. There are voiceovers from Walt and also from another announcer (probably when they couldn't find an appropriate comment from Walt). The promotional video is more "in your face", with very enthusiastic voiceovers. In other words, the promotional video is trying to sell WDW to you, while the park video is meant as a momento from your trip.
Both are excellent, if you enjoy images from WDW.
Also, as others stated, the park DVD has the Walt Disney Story. I think its the same as the movie shown at the end of the Walt Disney attraction in MGM, minus the Michael Eisner parts. I'm not positive though, since I only saw the film in MGM once. Regardless, it is EXCELLENT if you are at all interested in Walt. I'm not sure if the Walt Disney Story is included on the in-park VHS version, but perhaps somebody else can verify.
If you like to view your WDW videos repeatedly, the DVD is a must. VHS tapes wear down with each use, while DVD's do not.