The way you can tell which you should buy is by where you download from the most. If you go to Katz or any warez forum and more than 70% of the stuff you download comes from RapidShare, get a premium there. If it's MegaUpload, get one there. Wherever it is, that's where you should get it since that's where you download from the most. IMO, I like RapidShare to download (only). But if you download from another mirror a lot, buy it from there.
Think about it this way. You buy a premium of let's say MegaUpload but then you realize (oh shit...90% of my downloads are from Hotfile). Well guess what...you just fucked yourself in the ass. That's why you only buy a premium from where the majority of where you download from.