I'm fairly certain that SC takes whatever you give it and makes it a 128 mp3 for streaming.I believe I read that on their site. Exposure is a prime consideration. Soundclick and Bandcamp seem more "buried" on the back shelf with respect to the web. If you aren't tied into a decent search engine somewhere non one at all will hear the music.
While I think all of them are basically musician clubs with occasional non musician listeners, some fare better than others. Some would say SC isn't really that out here either. It probably isn't when compared to YouTube. I've been slow to move my music to YT, but it's probably a really good call if you're looking for exposure. YouTube is the new go to for most younger music listeners and a bunch of older ones too. If you are good on camera, have a likeable personality and something to offer you can really get good exposure there.
Reverbnation has way too much candy for me in terms of stuff musicians can spend money on and never see a return from any of it. I'm signed onto it, but haven't been there in ages. Too depressing. I guess if your a band or act travelling and have a fan base it's a much better thing because you can give a shout out to the followers, sell CDs etc. You could probably do the same things without them.