Yes, the Arc LS is slightly underdriven if I remember right so it should last forever. You can expect the shortest life (though still really long) out of a white LS because of the phosphor coating. On white LEDs (think 5mm Nichia) the coating degrades relatively quickly and there is a loss of output efficiency as a result. Lumileds (who makes the LS) claims that the way they assemble the emitter prevents this problem.
Colored LEDs are generally more efficient than white. This is mainly due to the phosphor layer in the whites which loses some energy in the conversion process (they use a blue LED with a phosphorous coating that converts it to spread-spectrum white light).
I think red LEDs are the most efficient in actual light output, but our eyes aren't as sensitive to those wavelengths as we are to green.
I think the orangish-red LS is supposed to be the overall winner for perceived brightness (according to Peter G). In non-LS lights, the turquoise color seems to be the winner.
The LS HD gives you a more ideal flashlight beam with a more focused hotspot than the LD. The total light output is the same for non-whites (the white HD introduced a new process that is more efficient).
So... the winner for long life and brightness is either an orangish-red or a cyan LS (I don't have either so I'm going on what others have said) and a HD version will give you the most focused output. Most people seem to prefer white, though, because of the improved color rendition and beam quality. Yes, the phosphor coating has a good effect, too--it makes the beam smoother.
Someone please correct me if I'm going astray here...
- Russ