Majora's Mask is without a doubt more powerful. Like some of you have said, the Ocarina of Time has the power to let you warp and do other things to help you on your quest. Majora's Mask is powerful enough to possess people and make the apocalypse reality. Pulling down something as massive as the moon takes a lot of strength. On top of that, the demon that dwells within it is completely insane and very powerful.
Like the Happy Mask Salesman said, however, we will never know its true nature. Is it really pure evil, or is it just insane, and destroying Termina is just a fun game to it?