Still, LuaJIT is easier to obtain and run, as far as I know.
The tests I've done were only inside of Firefox 4 Beta and I've got 3.4 sec for 5M n-body on my slightly (30%) faster computer. I remember that traditionally the standalone builds of Mozilla JS were not giving the same results as the one in the browser.
A message for Mozilla devs, in case anybody reads: be aware that people do try to measure how fast JS is from the command line and let them get the really last results! And use http://shootout.alioth.debian.org if you want to fundamentally improve your implementation of JS.
They can do better than use the benchmarks game - Kraken is one attempt they are making to do better. http://krakenbenchmark.mozilla.com/index.html
http://news.ycombinator.com/item?id=1754881
The benefits of the benchmarks game: allows comparison with other languages (like Lua) whereas as far as I understand their Kraken is JS only and browser only. Thanks a lot for your good work!