I can't speak for the Peak (though they have a good reputation), but the Arc is extremely tough, putting the Fenix to shame. I tried to like their 1st-gen E0, but it was made of softer aluminum, the finish wore off very easily, and it wasn't grippy enough when getting even a little bit wet (the knurling is poor, as the tube wears, its hard angles help less and less). The beam had more throw, and looked more white, much due to combining the yellow and blue sections of the beam. it also is a little brighter, and more efficient, but that only helps if you always are feeding new cells (see below). I think the regulation circuit of the Arc is ideal for such a small light.
The Arc gives you good voltage regulated light on any type of AAA battery (alkie, NIMH, LSD NIMH, CZ (yuck), Lithium), with 4+ hours of good light on a new primary cell; you can tell both by output and tint-shifting (it gets more "angry blue" as it dims) when it needs a new battery. It gives you hours, even on NIMH, to find a new cell, and will take to used cells (like scavenging from remotes and such) very well. If you keep it on, it will keep on until the battery is slap dead (it needs 0.9V resting or so to turn on, but will go down to something like 0.4V before it quits, IIRC). The Fenix will give you only minutes of indication before it is too dim to use, even though it does get more total light from a cell.
Runtime graphs of the original Peak Matterhorn make it look like a plain voltage gain circuit, dimming quickly, but offering a very long tail. They may do differently, now, or have an option for better regulation, if you call.