I've played around with both generations of the Lepton sensor and the 3.5 is really nice: factory calibrated so you can directly measure temp without elaborate external calibration and a built-in shutter.

Thermal sensors of this type are huge and expensive. At work I also have a Flir with 1024x768 sensor, super expensive with a massive germanium (or something) lens. One of the problems for handheld applications is that you always need two separate optical paths since the lensing is totally incompatible. So you end up having to register the two images to each other, which can fail when it's darkish or the image doesn't have a lot of edges.

They used a very crappy wordplay to hide the pixels for unknown reasons.