which is the best was to learn? - assuming that there aren’t dual subtitles available.
I’m learning german and I’m watching movies with german audio and english subtitles, because usually I can’t find subtitles in my native language… once I build up my vocabulary I will use german subtitles. This approach worked for me with english.
It depends. When I was a child, the whole country watched american movies with Spanish subs. Nobody learned English that way. We just read the sub. It was a reading experience of my own language. Thus, it was useless for us.
I tried watching Polish films with Polish subs. For me it is/was a reading experiencie. A poor one because a book is a much better one. Now I watch Polish films without subs. It is harder. Sometimes I do not understand at all. However, I know that this is a real listening practice. Currently, that is the why do I do Polish and French. Good practice is to watch serials. They are shorter, easier because the practice without sub might be exhausting.
for reading = I use a book with/out Lingq
for listening = No subs.
There are ways to extract all the subs from a movies and read them before watching the movie. The process is well described in Youtube.
Also, if you have only the audio, you can use WorkAudiobook to practice listening of short segments . I love this one and I am using it for French.