The Photon is very bright with a brand new battery, but if you leave it turned on continuously for 5 minutes or so, it dims considerably.
The Arc is regulated, and will maintain the same level of illumination for the life of the battery.
So far as batteries are concerned, those coin cells in the Photon are expensive-
having had the silly thing turn on via the switch while in my pocket (twice).
I bought an LSH premium second from Arc for $50 <img src="/images/graemlins/blush.gif" alt="" />, and it is my EDC at this point. Incredibly bright light, I am afraid to fool around with one of the newer lights, afraid that I would want one of those too!