I've got a Zoomed 55 gallon on my glass tank and a ExoTerra on my tub. The Exoterra seems to fluctuate (cool down) faster when my t-stat kicks off. The Zoomed seems to stay warmer longer. The Zoomed seems to be a little more sturdy/rugged than the ExoTerra.
I don't want to come across as pretentious, but you do have your UTH hooked up to a thermostat, right?