If anything Apple should buy Spotify. It is amazing, by-far the best thing I've ever paid for and it is the future. iTunes Music sales are on the way down, I haven't had a chance to try iTunes Radio yet b/c it's not available in my country but at the end of the day it is a radio, not a streaming service. Spotify is amazing with its social aspect, playlists, offline mode, your music mode etc... I agree, its music catalogue could be better but it's on its way.
Beats on the other hand isn't even so much top quality that it can add something to Apple technologically. It is all about branding and marketing moves with the logo and making the beats headphones "the cool" What will Apple gain out of it?
Spotify is the answer I tell ya.