It's brighter, and definitely whiter. To be honest though, I think the Arc is a better light. The Fenix is brighter at first, but you'll only get about an hour runtime from a NiMH rechargable. Which is OK if you don't mind recharging. Runtime kinda sucks with an alkaline. The Arc however will give you 5 hours of useful light from any alkaline. Long after the Fenix dies out (with any kind of battery) the Arc will keep on going. For me, that's more important than initial brightness, since I carry a larger, much brighter light with me anyway, and the light on my keychain is more for back-up or specific uses.
I'm not fond of the angry-blue surrounded by yellow-puke tint of the Arc, but other than that, it's one of the best values in keychain lights around. Myself, I have a CR2 Ion on my keychain, but these are 3x the cost of either the Arc or Fenix.
IMO, the best all-around keychain flashlight is the Jil (unfortunately they're not available at the moment, while updates and revisions are being made). There's a few different kinds. The 1.3W is larger and regulated. The DD (direct drive, no regulation) is smaller, less expensive, and although it's not as bright as the 1.3W, it lasts a looooooooong time. Like 20 hours on a lithium primary. Theres 2 versions of each, TWOH bin (works best with lithium CR2) and the TWOJ bin (works best with lithium ion rechargable RCR2s). Then there's the Jil Intelli which'll take either battery, has 5 user modes with selectable output levels, strobes, etc. Prices range from $65 for a TWOJ DD to $105 for the Intelli.