They are both great bands in their own respect. I personally like Dream TheatER over Katatonia. But, I enjoy a lot of Katatonia's music. I just don't know how there's a thread even remotely asking which is better. You can't compare a band with one of the best drummers, keyboardests, guitarists or bassists in a band to a more simplistic band. The Sepultura or Phish comparison maybe a bit outside the box, but Dream Theater vs. Katatonia isn't in the same ballpark either.
Maybe Dream Theater vs. Symphony X and Katatonia vs. Nightingale would have been better comparisons to which one might prefer.