User blog comment:Bluegear93/What ending did you choose?/@comment-24058081-20120503081834/@comment-4967682-20120503232651

'Destroy: Could potentially kill everyone and everything in the galaxy. Plus, Mass Relays are destroyed, other species can't go home."

It only kills the geth (and that's a really big maybe judging by developer statements), Synthesis destroys the relays too and effectively rewrites reality without anyone's input (Control 'merely' shuts them down), and Destroy is the only ending where we actually see the Reapers themselves destroyed. Only, unlike Control, you didn't even deal with the intelligence controlling him; you just have to take him and the Reapers at their word that they won't come back, rather than Control or Destroy them.