I've encountered different stories about the availability of content, but I think it really boils down to what sort of music you listen to. Personally, I find Spotify has more of the things that I listen to - but like I said I've talked with friends (that have different taste in music) that claim the same about another service.
That said, I think everyone I've talked to agrees Grooveshark has the best coverage, but everything there is so disorganized (and nobody is really sure of the legality).
Grooveshark is legal (AFAIK), but their business model is somewhat sketchy. Content is peer-to-peer sourced, and if you don't want your music on Grooveshark, you'll have to file a DMCA infringement claim.
