I use a FLiR type camera in my M3. I have it mounted in the front lower grill and have it display on my in car monitor to see deer, cops, etc. Have never received a ticket since installation.
I prefer it for many reasons. Sure, you can do what some budget people do, and use Nightvision, but amplifying the light around you makes and light source devastating. The thermal, while not able to work through glass, shows everything, even behind some objects (like bushes, the like). the detail is not there, but in reality, you are looking for more positional awareness type things, not always absolute identification.
With NV, unless you go Gen3 you'll need an IR torch. With thermal there are situations where contrast is obliterated as in you cannot see anything except an object with higher heat or cold signature, hotter or colder than terrain. Such as just after a rain on a cool day or night. I've got both but my NV isn't Gen3. Once I picked up thermal, my NV became a paperweight. Quality continues to improve and pricing continues to decline for thermal. If you decide on thermal, pick a unit that doesn't require focus adjustment as in once you set it, it's good to go at a mile or 30 yards, no focus required. I'd also reco a unit with a low native zoom, like 2X or 3X tops. I also don't like recessed buttons and complex menu navigation. I won't name what I use, but it uses turrets as opposed to buttons and the menu is simple to navigate.
That's my experience with access to only civilian market units.
Night vision is cheaper, but not critically. I would personally prefer thermal rifle scope for 1999$ (from www.atncorp.com/thermal-scope-thor-hd), meanwhile the cheapest NV googles cost about 1699$ from the same manufacturer.
The conditions that you are going to use the device in matter greatly too. The fact is that thermal imaging device will come in more than useful when it is complete darkness outside, while with minimal lighting it is best to use a night vision device. However, you should keep in mind that the night vision device is almost useless in the woods or other dark areas.
To compare the two devices, I would advise you to browse through the comparison between the best budget night vision scopes and the best budget thermal scopes. The author reviewed the way the night vision and thermal imaging recognition technology works in detail.