"Moving" MP3s from one mac book to the next?
By default, iTunes/Music copies audio files to /Music/iTunes. If you've got 15 GB of music in two locations, then you've probably got them duplicated already.
When you get a new Mac, you can use Migration Assistant to copy all your files and settings, and the process will be handled automatically for you.
If you don't want to use Migration Assistant, you can just copy the 'iTunes' folder inside /Music. The new app will update its Library database will all your data.