If you were to compare a more traditional film version of Twelfth Night to She's the Man, which would be closest to the actual spirit of the play?

It's difficult to say - I'll consider 'She's the Man' alongside the most famous film of the play, directed by Trevor Nunn in 1996.

Nunn's version is not entirely 'traditional': it is set in Edwardian, rather than Elizabethan dress, and it makes several cuts to the text (including adapting some sections of the play into montage, such as Feste's song 'O Mistress Mine'). Nunn also adds a prologue, written by Nunn and not Shakespeare, which details the storm and states explicitly the background to the story.

Particularly in his use of Feste, who knows from the beginning that Viola is really a woman and not a man, Nunn makes bold interpretative choices (you can support this reading of Feste with the text but it is by no means the only one). Yet  with its historical setting, with an emphasis on well-spoken, lyrical acting, and with its casting of largely British theatre actors rather than 'film' names, the film is 'traditional'.

She's the Man is an update, an entire rewriting of the play, preserving only the drive of the story, none of the language, and none of the details (a rewriting similar to that Shakespeare often carried out on his sources). Perhaps, in one sense, then 'She's The Man' is the sort of thing Shakespeare would do: certainly, its modern, gender-bending comic spirit is close to the spirit of Twelfth Night. But Nunn's film is undoubtedly closer to Twelfth Night as Shakespeare wrote it.