I'll have to put my vote in for Disturbed.
Disturbed has powerful lyrics, such as Inside the Fire was about the singer's girlfriend committing suicide. While their newest album, Indestructible, was so sellout it hurts, they are still the better band in my opinion. I like almost all of their songs, while System of a Down I like Chop Suey and BYOB, but that's about it. I also prefer Disturbed's singing. While the SOAD vocalist's singing is indecipherable, Draiman's singing is understandable and more downright powerful, melodic and also able to be more staccato, almost like it's part of the music. Disturbed is also more my style of music, period. SOAD is a little heavy for my tastes.