My *guess* is that IR would be much better since there is a much higher photon flux at IR than visible light.

Another thought: diode lasers at 1064nm (invisible IR) are very efficient at turning battery power into optical power, even at high output levels. Even if 1064nm LEDs don't exist or aren't efficient, taking a 1064nm diode laser and putting a diffuser on it should be visible to NVG at least 50% further away than a green LED at the same power.

Edit: Silicon sensors are usually very sensitive well into IR, and I assume that SAR NVG's don't filter out the IR since they want to notice hot engine parts etc. So I'm assuming at 1032nm light source is at least as easy to see in NVG as visible light.