Lua has been able to make major, reverses-compatibility breaking changes to improve its design in ways Javascript hasn't. Where Javascript has "The Good Parts" and incrementally improving implementations, Lua has been able to fix things and evolve.
Lua (without JIT) is also one of the fastest non-JIT, non-native compiled languages there is. LuaJIT is also one of the faster JIT languages. Now, I'm not sure how it compares to the popular JS JIT engines, but from what I've read, its very hard to beat LuaJIT for performance.
"One thing Mike [Pall] didn't highlight: get a simpler language. Lua is much simpler than JS." - Brendan Eich (http://lambda-the-ultimate.org/node/3851#comment-57671)