Wow...kinda unusual on this forum to see discussion of one's opinions outside of the lighting world... (and I think the forum has been better for it).
As to this question...
Ultra vs. ARC (AAA)
Much depends on your needs.
I own an Ultra, ARC AAA and a Photon for this "single LED" application.
I use them for different things.
I used to carry an ARC on my keychain but have gone to the Photon III (white) instead. It is smaller, lighter, more scratch resistant, less "scratchy" (from the ARC's sharp knurling) and more versatile than the ARC (P3 has modes). My ARC sits on a shelf now, except when I go camping when I trade it for the Photon on my key chain.
Yet...I think the ARC is a great light!
It's all about usage.
My keychain light is carried for small spots of light, here and there, now and then; keyholes, play programs, looking under my desk, in the glove box, etc. What I refer to as "occasional use". Which this usage, the coin battery will last 2 yrs (or more).
If I used it all the time, as my primary light walking around at night, the battery would last far less as long and the inexpensive, easy to change AAA would be way more of a factor in favor of the ARC. That is why I carry it when I'm camping (on my keychain).
Although, when I'm camping, my Ultra (or regular Blue-Green Infinity which I like just about as well) is always in my pocket or around my neck on its very nice lanyard (for hands free). The Ultra is brighter with a broader, smoother beam and with a longer lasting battery. The Ultra also comes with a nice clip that works great for clipping to a ball cap also for hands free work. The Infinity also will take a lithium AA for cold weather work. I got mine from TTS ($16).
So...I use two lights in this "space". The Photon for on the keychain and the Ultra for in the pocket or around the neck or on a cap.
But...(having said all of this), if you can only or are only going to buy one small LED, you CAN NOT go wrong by buying an ARC AAA!
My ARC just sits on the shelf due to having two other lights that fill various needs better for me.