There is only one question: Is a media agency best positioned to deliver a music partnership or do you need a specialist agency? I think the answer is you could do with both. Media agencies have muscle, budget and the ability to see beyond hype, specialist agencies understand music strategy, connections and contracting.
By now you really should know a lot of this stuff already but it never hurts to read it with pictures, graphics and formatting. I don’t really need to add anything to this as it is pretty self-explanatory.