First the obvious answer *buy them all!*
Yeah I'd have to say the KL1, for a couple reasons. First you have a E2 so it's cost effective. I can't recommend the A2 because it's not released yet. It might be the best light in the world but it doesn't do anyone any good till it's on the market.
Second while the L1 may be slightly brighter than the KL1 you get ~4.5 hours of runtime at full brightness instead of 1.5 hours. Granted you use more batteries but it's basically the same sized package. You do get the low beam with the L1 but if you can find room to EDC a L1/E2 you can find room for a E2 with a KL1 and a arc AAA on a keychain or jacket zipper. While it's nice to have a all in one light that also means that if that light fails you may have no backup. If you loose that light you've lost your low and high power option.
There are really advantages either way but myself I like redundancy. Not to say that you can't carry a arc AAA and a L1 too. Having an E2e with a KL1 gives you a high power option too, you use the 60 lumen bulb, put the KL1 on and have 4.5 hours of ~17 lumen light and have what 40 hours of Arc AAA light.
I admit the L1 beam is impressive and a runtime of 1.5 hours at that brightness is pretty darn good, but it's still nearly the same size as a E2. I mean you can get 75 minutes out of a E2e with 60 lumens in the same package.
If it was the size of a E1 I'd probalby feel differently. Then it would have a heck of a size advantage over the E2 with a KL1. Also I think the LED low power option is a lot more valuable with the A2 and it's 60 lumen beam than the L1, at only 20 lumens there's not too many times that would be TOO bright even for close up work, unlike a 60 lumen beam.
The ARC LS is a option too, I mean it may give up a little to the L1 in brightness but it's almost half the size. It's pretty spendy though.
I guess it really comes down to your specific needs and specific situations you use your light for. I mean if you want to carry only one light and do a lot of low light close up work the low option of the L1 is probably nice, and if you don't use the 20 lumen beam much it would probalby last a long long time on a battery. If you were using the 20 lumen beam mostly you might value the E2/kl1 with it's 3x longer runtime, it takes more batteries but you'd have to change them 1/3 less.
Buy em all, try em out, then sell the ones that don't fit your exact needs
There's certainly no problem selling surefire or arc stuff.
Todd