JohnyKwst: the standard advice on CPF: Buy them both!
The Arc AAA is bulletproof, and very small. It will serve you well on your keychain, and is small and inobtrusive enough that you will likely carry it everywhere. No flashlight will do you any good if you left it at home when you need it.
The Fenix is small, but a bit too large for keychain carry. It is much brighter than the Arc AAA, and throws a *lot* farther (the Arc is more of a close-range flood light). The build quality is good, but it's too new to assess long-term durability. For what it's worth, I let my 3 year old son play with my Fenix, and he's been very rough with it, with no noticable ill effects on the light (other than a couple scares when he disassembled it, put the battery in backwards, etc. Nothing that couldn't easily be fixed.)
The Fenix L1P's beam compares very favorably with my Arc LSs. Actually, I prefer the Fenix, since the Arc LS used an optic (as opposed to a reflector), and the hotspot appears square. It's not a functional issue, but I find it aesthetically odd.
I agree with Chrisse that if I had to choose between a Fenix L1P and an Arc LS, I would go with the Fenix even if the cost were equal (geez, I paid about 2.5 times for an Arc LS three years ago as I did for the Fenix). The main reason for my preference is the ability to use readily available AA batteries, as opposed to CR123s.
Everyone has different preferences, however, and I would say most CPFers would prefer the Arc LS because of sentiment, and the cool factor.