The most stupidest message I've ever seen in a movie

The portrayed in the movie is the same message that has been portrayed since the beginning of Hannah Montana. She chooses to live a double life in order to have a normal life but still be able to make music for her fans. It's never supposed to be considered lying, she just wants to be able to not be stalked by the paparazzi and be able to live a normal childhood/teenage years. If you're a fan of the show, you should realize this and not try to pick at the movie for her "lying" to her fans.

Also, there are pleanty of messages throughout the movie, as there are throughout each episode of the show Hannah Montana. Imagine this movie as one long show, that has a bigger budget. All of the fame and money were getting to Hannah, so her dad decided to bring her back home to Tennessee to remember who she really is.
After spending weeks back in Tennessee she forgot her life as Hannah Montana and fell in love with the life she had in Tennessee, but once she was to perform again as Hannah, she realized how much she loved it also - thus the choosing just one. She was sick of living a lie, but was torn because she doesn't want to quit singing, but also is worried about losing her privacy.

As for the tagline for the Hannah Montana Movie, as you put it "She always had the best of both worlds, now she has to choose just one?" Would you prefer if it would have said "she's always had the best of both worlds, now she has to choose just one, but its ok becasue her hometown will keep her secret, so she'll be able to still live a double life." The tagline of the movie is just that.. a tagline to a movie. It is supposed to draw in a crowd - as it did for you. If it gave more detail about what her decision would really be, then it would no longer be a tagline, but a spoiler.

Also you say that movies are supposed to take characters on a journey, this would be the character arc, and Miley goes through a very obvious character arc. In the end of the movie, she performs a song called "The Climb." If you listen to these lyrics verses the lyrics of other songs off the record such as "The Good Life," you'll see a difference in subject matter that goes from bragging about all of the expensive things Hannah gets to buy to The Climb which talks about overcoming struggles, uphill battles, and talking about how its not about "how fast [she] gets there, or what's waiting on the other side, its the climb." The Soundtrack even has a character arc.

At the end of the movie, Miley even says she loves being Hannah, but she doesn't want to lie anymore. That is why everyone is so willing to keep her secret, so she can be the dream of every little girl.

She chose Hannha. But she is a new person after what she went through in Tennessee.
Her decision she had to make stemmed from the car ride with her dad when he told her that if she didn't change how she was acting she would then have to choose. But alas! She changed how she was acting.