It's your choice. Blowing up Megaton kills everyone in Megaton, except for Moira from Craterside supplies, she will become a ghoul and upon return to the megaton ruins she will strike a conversation with you. She will then proceed to Underworld where you can continue with the wasteland survival guide. Also you will get a apartment and a large payment if you decide to blow up megaton.
If you decide not to blow up megaton, then you will just get a house in megaton.