Having used both IR and NVG regularly (together in the same aircraft), the major difference is that NVG will only detect light. No light, nothing in those green tubes. An IR will detect heat without light so a baddie/lost person behind a bush who is dark will still be found (as long as not shielded completely by a solid object). If a person has a light, the NVG will pick it up much quicker and the IR can be talked onto target.
With regards to height, slant range detection is what matters and somehting like a cell phone light or key pad is very hard to pick up more than 500m away on NVG. Note the video clip is actually using a torch for added effect.
Using both together is great and not much gets through the net.
