To me, what makes a musical great is the emotions stirred by the music. That is not true with a movie. It’s actions and dialog of a movie that make it stirring and the music does an important part to set the mood. The greatest movies meld everything together.
When you watch a musical you do it for the music. The plot and actions are important, but you’re really hoping to hear some awesome tunes that you can enjoy and either make you laugh or cry. This is where movies and their musical counterparts differ.
A subpar movie can make a great musical of the composer and song writer pick the right array of songs. A lousy musical probably will make a lousy film, but you don’t see too many musicals to films anyway. To me a prime example of the subpar movie making a great musical is Legally Blonde. I thought the movie was cute, but had a lame premise and predictable outcome.
None of that mattered with the musical because the music was so catchy and fun that you overlooked many of the other points. No one ever leaves a theater reciting the dialog, but more than a few have left humming some of the memorable tunes. For me, the musical will always win over the film, but that’s just my taste.