i, like ken, used to be a longevity freak...now it's become more refined as to be an efficiencey freak...
i bought an LE last month and i'm glad i did...it is a good light, pretty bright for a single LED...the construction is good quality, the HA-III finish is most excellent...if you want a fairly bright (a bit brighter than the mag solitare, but doesn't throw as far) white LED light, this one is great for it's size...
i've had a green LED infinity (supposedly their brightest) for quite awhile (over a year) and it was the most used light i had for a long time...i don't seem to need (or want, even) as much light as other folks...i find this light to be great for close up work where color isn't an issue...it preserves night vision very well -- for reading maps, looking through a ruck-sack, navigating the house at night, etc....very sturdy, solid construction...the run-time is an excellent side-effect...
from an efficiencey stand point, LEDs are much more efficient at lower voltages...the infinity probably drives the LED close to specs where-as the ARC overdrives it to it's maximum...over-all light (meaning throughout the life of the battery) is going to be much better from the infinity...
they are both good lights, it really depends on what you plans are for the light...
all-in-all more people sem to like the arc's brightness compared to the infinity's run-time and i would probably recomend that to someone trying to decide between the two...