(+)Ruby has a large user community and consequently has a large and various collection of libraries and documentation.
(+)One Ruby web application framework, Rails, has recently gained wide name recognition.
(?)Rich syntax with many non-alphanumeric characters used.
(-)Ruby is not easily embeddable. However, MRuby is very easy to embed.
(-)Some functionality is still being built. Documentation is concise.
(+)Lua has two web-application frameworks, Kepler and Aranha. Both of which continue in the spirit of Lua. Light, extensible, and portable.
(+) 'One of the main reason that won me over to Lua from Ruby was that Lua already had a very good VM which allows Lua programs to be compiled and also to execute a lot faster than Ruby (x2 in most cases).' This might change with the upcoming "MRuby". MRuby is a ruby subset targeted at embedding on small devices, with its own VM. The development version is publicly available since May 2012 on Github.