Buy the waterproof one. I'm on my second Thermopen as my first one died, and this one just went way out of range so I'll need to crack it open and do a manual calibration.
Dampness and humidity killed the first pen, and I suspect the same for my current one. If it gets a little damp or steamed up then it causes a film to form on the circuit boards which short it out. For that reason alone I would buy a waterproof pen, and I might have to if my current pen can't be fixed.