Regarding the price difference and design, this is a copy/paste from Thermoworks' website of a customer's comment, and their response:
"Prefer the Thermopops
I have the Thermapen and two of the ThermoPops. I now only use the ThermoPops. I think the ThermoPops are as accurate and as fast as the Thermapens but they stay lit longer than the Thermapen. About every time I picked up the Thermapen the display had gone blank and I had to put the cooking utensil down and close and open the Thermapen to get it lit again.
I am going to buy several more of the ThermoPops as gifts for family members.
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September 20, 2016
Mack
Response from ThermoWorks
We are glad to know that you are happy with your ThermoPop. We do feel that we need to clarify that a ThermoPop (which uses thermistor technology) is not as fast and as accurate as a Thermapen (which uses thermocouple technology).
The Thermapen is almost twice as fast (2-3 seconds vs 3-5 seconds) and because it is a thermocouple, once it finds the initial temperature, a Thermapen is able to display changes in temperatures instantly, which a ThermoPop cannot do.
Also, a Thermapen is more than twice as accurate as a ThermoPop.
* Thermapen is accurate to ±0.7°F (±0.4°C)
* ThermoPop is accurate to ±2.0°F (±1°C).
Also, it sounds like you may be comparing ThermoPop with a Classic Thermapen, which does require that you close and open the probe again after it turns off. The Thermapen Mk4, on the other hand, has auto-wake, which means that as soon as you pick it up, it turns on. It does not require a button push, as a ThermoPop does."