I'm mostly sticking with Rdio for now since it's the only thing that gets through my work's proxy. I've rarely been disappointed by Rdio's selection and mostly use it to browse new releases every tuesday. If you're more interested in backlog and have your own huge library, spotify may be a better choice since you can upload your own music to your online collection to fill any gaps they may have in their selection. Rdio's more about making social links and following other people to discover new music. I've been following like Spin magazine and a couple other people who I've discovered to have similar tastes, so when 40 new albums come out every week that I have no idea who's who, I can easily see on my front page what people have been listening to and I can start from there to find something I like. I've read Spotify and other services dont do "social" as well, so that's something to consider if you are interested in new stuff, but dont subscribe to a lot of music publications to know what's coming.