I think Jersey Boy movie will be better than the musical. I think because they are using a lot of the people from the actual musical it will come across the same and they can re shoot scenes and really make sure it is perfect will give them a advantage over the musical.
Film versions of musicals rarely match the stage versions. Sure, there is the opportunity to go on location to fill out the background in the film versions, but it often takes away from the focus on the music. Additionally, hearing the songs performed live gives an immediate visceral thrill that can't be matched by actors doing lip sync on film. Finally, I don't really have a lot of confidence in Clint Eastwood directing a musical.... did you see him in "Paint Your Wagon?"
The Jersey Boys movie is in competent hands with seasoned director Clint Eastwood, however film and stage performances are two mediums with very different atmospheres. There is a certain buzz within a room when you witness a stage production of a musical, that is rarely recreated in the cinema, it's an experience and makes it almost unfair to compare the two. I don't doubt the movie will be excellent, but don't expect it to be the same.
No, the Jerseys Boys movie cannot possible be better than the movie, because the play was amazing. They used so many original songs, like "Sherry Baby," and "Big Girls Don't Cry." I also liked the plot. They used talented actors who really portrayed the story, in a personal way that the screen can't copy.