• I2 NV lets you see in the dark, so camouflaged animals are still camo'd hidden in grasses, just displayed in shades of black/green/white.
• Thermal lets you see through the dark grasses, and animals stick out from whatever background (or foreground) camos them.
Back when thermal was laggy and pixelated, I used to like to scan with handheld thermal: better detection for spotting. Then I'd get on the gun with a green clip-on/scope setup: better resolution for the fine crosshair work of shooting.
Now that 640/fast refresh thermals are common and cheaper, (and just better lower-res ones too) I've kind of moved to thermal-only, but hang onto a pair of PVS14s to helmet mount or loan out to a newb when hunting, etc. Seeing hogs in tall grass, differing between dark animal shapes in front of colder waterholes, contrast on coyotes in brown fields, etc. all make thermal the better choice for on-gun use... pretty much everything except fast moving and driving.