Train is better than the Megabus. Megabus is good if you're lucky enough to be able to get the bus at one of the few times a day they depart... Trains depart every fifteen minutes or so at peak times, and run all night!
But yes, as Becca says, if you plan to go to Campus, get a ticket to Flamer. It usually costs the same as a ticket to Brighton, but saves extra hassle. And you can still get out at Brighton for a bit with that ticket too.
EDIT: Megabus do have a service to Falmer campus, but National Express you have to get off at Pool Valley, by the seafront. But the buses to campus ain't bad at all.