Trocky, there are two basically different Arcs, each made in a few variations:
Arc AAA: tiny keychain light (size of Mag Solitaire), powered by one AAA cell, has a DC-DC converter to boost the voltage to run the ~9000 mcd white Nichia LED. It will run for four hours or so on a new alkaline AAA, or over an hour on an AAA that's too dead to run an incandescent light. This is a beautiful little light and everyone should have one. The "standard" version is 20-25 bucks. The "limited edition" has a hand selected LED for smoother color and maybe a touch more brightness, but costs quite a bit more and (my opinion) isn't as good a value as the standard version (get it if you're very fussy or just want "the best"). Versions have also been made with different colors of LED and different finishes, though most of those aren't currently available. Note that the AAA's brightness is like a typical penlight, enough for close-up tasks or seeing where you're going in the dark, but it's not a long-range searchlight.
Arc LS: This uses a one watt Luxeon LED and is as bright as a typical 2D flashlight. It uses one lithium cell but there's a 2AA battery holder available for it as an accessory, so you can use rechargeables. With the 123 holder it's a bit smaller than the Surefire E1e/KL1 combination. Opinions here vary as to which is preferable. The Arc is a smaller light with typically a better LED, but the Surefire fits into a big system of accessories. The Arc LS also comes in a more expensive "premium" (LSH-P) version with a selected LED. Right now the Luxeon LED's are bleeding edge parts and there's enough variation between them that the premium Arc will really get you a brighter/whiter light (i.e. bigger difference than between the AAA's), so you have to decide if it's worth the extra cost.
Between the two of these I'd say you need an AAA since you can put it on your keyring and always have it with you. The LS is a very good light too but there are various other options you might choose for a pocket sized, medium powered light. (Examples: Surefire E1e/KL1, Minimag/Badboy, or even an incandescent like a PT Rage or an E2e with MN02).
Regarding the E2e/KL1 combo: the KL1 has the same 1W Luxeon LED as the Arc LS so it serves about the same purpose. The E2 body has two 123's so it has longer runtime than the Arc, at the expense of increased size. Do you really care about swapping out two batteries after four hours, instead of 1 battery after 2 hours? So I'd say the E1(e) makes more sense than the E2(e) for attaching a KL1. But the E1/KL1 combo is still bigger than the Arc, and Arc seems to select LED's more discriminatingly, so between the two I'd take the Arc. The E2's main advantage is its much brighter MN03 incandescent head, or the coming KL4 5-watt LED head (that needs two 123's to run).
So: if you want a bright LED light, I'd say get the Arc LS, and get the 2AA tailpack and some NiMH cells if you want economy running it. If you want an even brighter incandescent light, get the E2e/MN03. And if you want a great combination, get both, they won't overlap like an LS and KL1 would.