msinc brought up many of the drawbacks to thermal. I do not know if this matters to you but many are not rated for larger rifles. Like semiauto 308 rifles.
A huge draw back to NV is shadows. NV needs light to work and they just do not have the ability to see into most shadows. I have lots of trees in my yard and many things use the trees to move through my yard. Even with a IR flashlight it is hard to look into trees and see anything. This is where thermal excels. Even if you just see little heat spots you know something is there.
If you were going to purchase a thermal scope which would you buy and why? Pixels, Magnification, brand, etc.