MegaPowers just activate, they are extensions of a person's natural abilities. Power Traits require a successful action to generate and skill to activate.
MegaPowers generate fatigue upon use for three impulses. Power Traits generate manna every time they are activated.
Power Traits are good for spells, psionics, and other magics. MegaPowers are used for super heroes, supernatural and paranatural beings (werewolfs, vampires, winged folks, aqautic Atlanteans, and people who fly, through powerbolts, and have a recent string of hit movies). Mega Powers can also be used for cyberware, the hyperskills of an immortal, and so on, as long as the powers are effectively an innate part of the character.