I’d say Spotify because of its seemingly better selection, based on my limited use of it today. Their Mac app’s interface is horrible compared to Rdio’s, but their iPad app is good. Pricing’s about the same, except that Spotify has a free/ad-supported tier. I’d say Rdio is a nicer service, but it has pretty serious rights issues (in Australia), and what’s the point of a nice UI if there’s no music?