Steve - wait it out a bit unless you have a pressing need for a luxeon light. The KL1 may not turn out to be everything it's cracked up to be. I'd wait until some reviews are in.
If you've already got the E2, then a luxeon bezel would be a handy add on, if you don't mind footing the bill for those expensive lithiums.
OTOH, the ARC runs on several different battery configurations, is regulated (is the KL1 regulated? Al? I havn't been keeping up with it...), and is field proven.
I sincerely doubt that Surefire is putting as much research into Luxeon LED lighting as Peter is doing over at ARC. That's just about ALL Peter is doing. Surefire has a lot more than Luxeons on their plate, so there may be bugs to be worked out when the KL1 first comes out.