I have the sticky type on a couple of tanks, and I've noticed that they are pretty inaccurate when the ambient temperature is quite different from the tank temp. Our house gets quite cool at night (in the winter) when the heat goes down, and warms in the day when the furnace gets turned up. The sticky thermometers do a lousy job when the room temp isn't close to the tank temp, or when the room temp changes dramatically relative to tank temp.
The in-the-tank thermometers are cheap and generally accurate, although I've never found any where the rubber suction cup lasts for more than a year or two. I've been using inexpensive digital indoor/outdoor thermometers for several years and they work quite well. Radio Shack sells a $9.99 model with a sealed temp probe that works well and has an easily to see readout. Most of these units are three to four years old and going strong.
HTH,
Jim