Soundcloud is indeed great for mixes and sets. They have valuable and unique content, yet the product remains very barebones and lackluster. Poor audio quality (still 128 kbps MP3 I think?), no concept of tracklists/timestamps and most importantly your playback position and queue isn't persistent. This is crucial for a medium used to listen to hours long mixes - as a user I expect to be able to pause and pick up my currently playing set later, on any device.
The lack of this feature basically forces me to manage my playback queue in a different tool, like a note taking or todo app. I estimate that I have countless unfinished sets in my history that I simply forget to finish since once you close your browser window/app the content is basically off my radar. Imagine Netflix not saving your viewing progress - it's absurd.
I think Mixcloud is the much better product - it just lacks the vast amount of content (they are gettign there) and numbers of listeners unfortunately. They also have a sound business/pricing model - although that may indeed be a barrier to listener growth.