If they're both for seven dollars, they're probably the more inaccurate kind... But go ahead and put up a link just in case you ended up with an amazing deal. A hand-held laser thermometer is the most accurate. They're like twenty dollars at Petco and they last a long time. The ones you stick on the side of the tank can be off by like seven or eight degrees.
Yeah, if you haven't had a thermometer/hygrometer in there I'm betting that's your problem instead of scheduling.