Hi Nastaran,
The Lepton is a small core unit that you will need to do all the work for - put it in a device, write code, test and deploy. Depending on the version of the core you can have a “radiometric” version or not, but you’ll still need to do some calibration.
Ax8, A400 (also A70, A700 etc) are both products, you can plug and play them onto your application directly and set up I/O etc using their web pages or FLIR software etc.
There is also a FLIR Photon, which is a small camera that is more finished than a lepton but you still need to write code etc (FLIR does have a GUI).
It really depends on whether you want to build your own solution or use a readymade unit off the shelf.