The two are fairly different: mod_ruby gives you access to the full
Apache API including access control and output filtering, while FastCGI
(at least in the least-common-denominator form that Apache supports) is
basically just a means of avoiding the process forking and
initialization overhead of traditional CGI.

There are pros and cons to either solution. Because it gives you so
much power over the entire HTTP request handling process, mod_ruby can
let you do things that are more difficult with CGI, like request URL
rewriting. However, it ties you to Apache exclusively, while FastCGI
handlers can be run behind any number of HTTP servers, including the
increasingly-popular Lighttpd.

Personally, I'm moving towards using FastCGI behing Lighttpd for a
number of applications, primarily due to its built-in load balancing.
If you don't need the kind of low-level power that mod_ruby offers, I'd
recommend staying with FastCGI for its simplicity and portability.

-rcoder