I have an electrical engineering degree (although I no longer work in EE so my memory is not 100%) so I'm fairly confident in what I'm about to write...
- TroopLewis, your original question was about "pulling 2A". Do you mean from the battery, or 2A through the LED? If you mean from the battery, then as other people have said, the driver plays a big role in overall performance and efficiency. But for the sake of this discussion, we could say "driver efficiency being equal...", which would answer your question better, correct?
- If we make the above assumption about drivers, and if TroopLewis means 2A from the battery, then the same amount of power is being delivered to the XP-G and XM-L. That is, 2A, at the battery voltage (which of course wouldn't change with different LEDs), minus the power loss in the driver and wires.
- As brted and okwchin said, the XM-L is more efficient than the XP-G. So given the same amount of power delivered to each, a little more is converted to light in the XM-L, but it might not be a very significant amount of heat (I believe a small amount of heat is equivalent to a lot of light, in terms of energy).
- About heaters: Don is right when he says all energy eventually becomes heat. Even light eventually becomes heat. When light hits an object, some energy is reflected back, but some is absorbed, and that absorbed energy heats the object. Dark-coloured objects reflect less light back than light-coloured objects. That's why a black car with a black interior gets much hotter in the sunlight than a white car with tan interior.
- ronparr is right when he says that heaters are 100% efficient, but let me add to what he wrote: all of the power you put into an electric heater will come out as heat, somehow. It may not all come off the heating element though. So if you put 100W into a heater, maybe 80W comes off the heating element. The other 20W goes into making the enclosure hot, the wiring hot, etc. If there's a fan, then some power goes into making air move, but eventually, the air slows down and kinetic energy is converted into heat. Ultimately, the fan's power ends up as heat, as Don implied. Another heater might make 90W come off the heating element, leaving less power to be dissipated in the enclosure, wiring, etc. (did you know that leaving a fun running in an empty room, with doors and windows closed, actually heats up the room? I could never convince my parents of that. The fan consumes power. All of the power eventually becomes heat. A fan cools people, not air. The layer of air next to your skin (the boundary layer) is heated by your body. If the surrounding air is cooler than your body, then the fan will remove the boundary layer, leaving cooler air next to your skin, cooling you. Of course, a fan will cool a room if you blow hot air out a window or door.)
- Be-Seen Triker, you said that the characteristics of the XP-G and XM-L are nearly identical. You're omitting one major property though: the forward voltage drop. If you compare the forward voltages of the two LEDs (someone posted links to the data sheets in a post later than yours), you'll see that the XM-L has a much lower Vf. e.g. at 1A, the XP-G's Vf is about 3.35V. At 1A, the XM-L's Vf is about 2.97V. Since power dissipation = voltage x current, then if trooplewis's original question was about pullling 2A through the LED, then the XM-L would be dissipating at least 10% less power than the XP-G. And at 1.5A, the difference is even larger: around 3.55V vs. 3.1V. But if trooplewis meant 2A from the battery, then my second and third comments above apply.
That's all. Hope I didn't sound too know-it-all-y. And I sure hope I'm right. :)