Mega was a re-thinked Megaupload trying to accomplish the same thing but they are very different as Mega is built with a different tech/business architecture to try avoid crossing the lines into illegality that Megaupload crossed.

The original Megaupload was built to take advantage of the market opportunity existing in the grey area between copyright infringement and personal sharing. They were not trying to make something illegal, instead they were trying to make something as close as possible to illegal while still being able to argue that they are technically legal, which they failed, as the powers-that-be decided that they crossed the line into illegality and persecuted them. After the failure of Megaupload they tried to do the same thing again with Mega, but armed with the new knowledge of what is now considered clearly illegal, what will get the persecutors knowing on your door, they redesigned the new product to not cross those new lines.

More specifically, Megaupload fell because it was provable that they knew about the individual instances of piracy, so they designed Mega around end-to-end encription, so Mega users can still pirate things but Mega can always claim ignorance as a defense. There was/is no difference in persecutor priority in regards to copyright (copyright infringement is still a top priority evil today), just a difference in what they can prove against those two different companies.