Dropbox is useful for sharing music files in a controlled and targeted way - you can allow access to a file for specific individuals, anyone with the link, or the whole wide world. The built-in music player seems to work ok. In my opinion, it is much better for the uses you mentioned than Soundcloud.
Soundcloud is kind of hard to figure out (my opinion, again). Also on the minus side, users need to have an account to download files - that is not a dealbreaker, but makes it less easy for VJR, for example - and I found the social media aspect of it to be useless and even irritating (regularly receiving and turning down "friend requests" from spammers). If I were trying to get the world interested in my music, though, Soundcloud would probably work better for that than Dropbox. Also, there's no reason you can't have both!