Crazy4 has the right of it. The CHE's tend to dry out things a lot and if the tank is a tall one they often don't get a hot spot (generally accepted 90-94) They can be use to try and lift the ambient temps up some but are not very efficient to do so.
UTH need some sort of control there are a lot out there the advantage of a programable proportional control (herpstat, helix, excozone) is you don't have to mess with it, set it where you want it and it takes care of the rest. The cheap on off type (zoo med 500r) can allow very high spikes if there is not some sort of intermediate space in between. I don't have any experience with the programable devices like a ranco so I can speak to it.
Rheostats can not adjust for room temperatures variations, they either need to be in a constant room temp or need to be adjusted everyday. Thermostats will do that daily adjustment for you.
Alex
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