I think the difference is that to run an Arc powered website, you would need an Arc box on the server side, which is what you would need for running Clojure websites as well. What ClojureScript does is generate and compile the necessary JavaScript which can be off-loaded onto any (or at least most) web hosting sites, including shared web hosts. Additionally, the use of compiled JavaScript gives you the benefit of leveraging client-side processing, as opposed to Arc or Clojure boxes which are server-side solutions.
But you are right, Arc is great too, and it ultimately boils down to individual choice. Arc is my first Lisp, and I like it above Scheme/Common Lisp, as well as Python and Ruby.
