Storm is the more powerful elemental.
She once saved the Earth by surrounding the planet with a gamma-powered weather shield. She took gamma rays out of a gamma ray gun and fused it with the energies of the world to create a shield around the Earth deflecting a world-destroying solar flare from the sun.
She has show the ability to be able to suck the full energy out of heavenly bodies, be it stars or planets, and has done this to millions of stars and planets simultaneously.
Her elemental abilities can adapt to control the natural energy systems and forces of whatever dimension/reality/space/planet she's in or on.
She once redirected the full power of Sienna Blaze. In that very same issue, it was established that Sienna has the power to split the Earth like a ripe melon.
I believe Storm to be the more powerful elemental, but Thor has more opportunities to cut up with his powers given he has a solo series and the Avengers tend to fight more powerful opponents than the X-Men.