I think they are all about the same. The all have a probe, screen and need a battery (included).
I leave mine on all the time. Had them for a year and haven't replaced the battery yet.
Amazon.com : Marina ThermoSensor In-Out Thermometer : Aquarium Thermometers : Pet Supplies
Btw, you can get replacement batteries really cheap on Amazon. It got 20 for the price I'd pay for one in the drug store. Brand name sealed in package, too.
I stopped by Petco today and they had that exact Marina thermometer as an opened package, with a reduced price, so I went for it. Seems to work fine. It reads right smack in the middle of my two glass/alchohol thermometers. One is 2 degrees above, the other is 2 degrees below. So I guess I have to assume that the digital is the closest to the truth? Really frustrating when you are trying to achieve a specific temp to get rid of disease. I guess I need to get some ice and make a glass of ice water and see which one reads closest to 32 F.
There are rare reports of temp resistant ich. I think it is more likely that your thermometer was the culprit. In any case, this is the very reason I combined Ich Attack with temp. If it isn't going away try adding Ich Attack to the mix. It is one of the safer meds you can use. I believe that it along with temp will be the knock out punch you need.