I'm focusing on Clojure now though I will keep an eye on Arc developments. Arc is really nice to play with but I agree that Clojure has the traction, even by the yardstick pg mentioned about what it'll take for a language to dominate (community, number of books, power of the language itself, etc.). It also has the whole Java library to support it, even though it has led to some uncomfortable syntax in some cases.
Additionally, I find Rich Hickey has given deep thought to his philosophy re programming and its challenges, and has applied it quite consistently over the years. There is a clear pattern in his approach to the development of Clojure and more recently to that of Datomic, with much of it coming out of the ideas he talked about years ago on the concept of simplicity.
