There is a big difference, actually, that is better/worse depending on your needs. Dropbox (at least right now) automatically syncs all files to the local disk

There is a big difference, actually, that is better/worse depending on your needs. Dropbox (at least right now) automatically syncs all files to the local disk of all linked accounts. This is often advantageous (the files are ready right when you need them), but for some things, such as a large MP3 collection, is unnecessary and wasteful. For that use case, S3 or one of the other synching services would be a smarter choice.