Though Spotify keeps being a good place to discover new, but mostly guaranteed music, in Soundcloud you will find new stuff. Always. Good? Lovable? Clean? Doesn't matter. New? Hell yeah
Superior algorithms + less ads than Spotify = SoundCloud forever (seriously, the auto-play never gets reckless, you can leave it playing forever it's that good)
Unique community and great platform for up and coming artists. Sound Cloud holds mixes and relevant music in ways that YouTube and traditional streaming platforms do not.