I agree with Lebron. I've got Hydor THEO's in all three of my tanks (a filtered 3 gallon and 2 unfiltered medium KKs) and they do a great job keeping them at a constant 80. I did have to do some adjusting on all three, setting the dial a little higher than 80. But once you figure it out, they're golden. No muss, no fuss.
AND the Hydor is smaller than the Elite, so it fits very well in all three of my set ups. One more note....Although it says the Elite is fully submersible, if you completely submerse in water above the little blue "globe", you may notice an olily sheen start to release into the water (from what I've gathered is a lubricant that is used within the dial). With my KKs, I like to keep them horizontal close to the bottom of the tanks. So I needed something I could submerse completely.
Coming from someone who has experince with both, I defintely prefer the THEO.
Good luck choosing one that works best for you!