I don't think either one is "great" by any means, but I do enjoy the drumming of both. they both have different styles and I think both became better drummers as the years went on with their respective bands.
I think that Lars became a better drummer over time than Alex did, but the band's style didn't always show that; then again, listen to 5150 and show me one outstanding drum track on it, so the recordings don't always tell the full picture of talents or abilities.
However, Lars is also FAR more hateable. As a fan of the band and fan of the music, I have no qualms pointing out what a dislikable douchenozzle he is.
I'll really have to think about it before voting.