Objectively speaking, the Nitecore EX10 has more useful features, but the aesthetics and durability of the Arc6 more than make up for not having things like realtime-adjustable brightness -- the Arc6 feels like it was made by your friend from high school who now runs a machine shop.
But, ultimately it's just a light and its job is to light up things, and they both do it very well. If you don't want to spend $250, and/or you don't care about aesthetics all that much, and/or the user interface is the most important thing to you, I'd get the Nitecore EX10.