You need a huge amount of hits before Megaupload pays out (and they're picky about acceptable sizes). Plus the payout is waaaaaaayyyyy smaller. You need 100,000 hits for MU to give $100, Fileserve can pay up to $25 for 1,000 hits.
Of course, I've posted a bunch of uploads on both Megaupload and Fileserve. Generally by the time a MU file has 100 hits, the FS mirror only has 2-3 hits