FCGI processes versus Mongrel servers

I can bet that running 10 FCGI processes is much lighter on resources
than running 10 instances of mongrels. what do you think? then why is
running mongrels is more preferable these days? Is there any work
going on to make rails thread safe?

We have had far better performance and overall system stability using Mongrels over FastCGI processes. If you used something like fcgid or one of the newer fastcgi implementations its better over mod_fastcgi, but we still have found, in several different big apps, that Mongrel is usually better than FastCGI.