I agree with CollegeBettas. An adjustable heater costs more, but it will be a better & cheaper investment after all. I used to use heating pads (non-adjustable) and had to replace them with adjustable ones. I can't be happier with my new heaters.
My betta just went into a new 5gallon tank about 4 days ago. I haven't found the heater I want yet but she has been active till till yesterday. Now she has been staying at the bottom of the tank in her tunnel. I managed to get her out of it once to eat and now she is back in. My house gets cold at night so I thought it was because she is cold. But she is still sitting at the bottom in the tunnel. Think maybe she is done with the excitement of a new tank and is just being lazy?
If you can swing it, I'd get an Eheim Jager heater. They're built solid, however, can sometimes be a little difficult to place in the tank because of how long they are.
Just remember to calibrate the dial when you first get it.