ROBU remarked "It's complicated" (NO LIE!) and also recommended this blog:
http://aristake.com/
Well...an hour later of combing through this site I am basically a gibbering pile of goo. Ari has been running it for at least 4 years and has exhaustively reviewed countless music distribution services -- some new, some older, some now defunct; and there are literally thousands of comments from people all over the world giving their experiences with them.
As noted; the answer comes down largely to: "it depends". CDs, streaming only or both? What are you willing to pay? How many services do you want to be on? How much accounting detail do you need? How many releases, and how many tracks per release are you talking about?
And on top of that are the intangibles about
: customer service stories, speed of paying royalties, upload bugs, speed of getting music on sites, etc. etc.
Basically all I'm going to say is: check out this guy's site and if you have the patience to trawl through megabytes of data you should be able to find an answer that works for you.