It is going to be the sensors followed by heat detection ranges. What I look at when viewing these different Thermals, by the way I do not own one, I first look at heat detection range followed by identity range, what is the range you can without a doubt distinguish this being a hog versus a calf. Second thing I really look for is sensors some are 640x480, 320x240, etc. This is the pixel resolution the higher resolution the better viewing, another is refresh rate in hz. Look at this like you are purchasing a new TV, you see all the 1080, 4k resolution, that is the crispness of the picture, but also look at refresh rate in hz, 60, 120, etc. That is going to be the difference of models along with the add on, like wifi ability, recording, recording with sound, etc. However, I own a Night Vision optic by sightmark and the number one item I would look for is the ability to add a USB battery pack. My NV will run through the normal batteries in 1.5hrs of use set at the highest IR setting. When I use my USB battery pack 10k ma, it will last 6+ hours even longer. Just something to remember, it does no good if you have to change batteries in the middle of a hunt in the dark.