YouTube, because you can upload audio in higher quality.
You can upload a file size of up to 5gb to soundcloud so it can match YouTube on upload quality... soundcloud though only allows streams at 128kbps so if you give the option to download said file - the person downloading will get the same quaility as the file that were uploaded.
I always think soundcloud is a little bit of a musician's circlejerk. So many users are just trying to promote themselves in a kind of 'like-for-like' mentality.
Youtube is used more by the general public I think, but it's easier to drown in it as there is more content to get yourself lost amongst