Ferret talk - Apr 2007 - Best Practices for Searching Multiple Indexes -- multi reader or combine?

Back in September (http://rubyforge.org/pipermail/ferret-talk/2006- 
September/001210.html) Dave said:

"Another use-case might be to have multiple indexes to speed up  
indexing. If for example you are scraping websites it is a very good  
idea to have multiple scraping processes. The best way to do this is  
to have each process indexing to its own index. You could then search  
all indexes at once using a MultiReader or you could also merge all  
indexes into a single index."

Does anyone have any recommendations as to whether one method is  
superior to the other? We have a system where we build multiple  
indexes from scraping websites and need to expose the indexes as a  
web application--the web app will only search the index and will  
never modify it. If we have 30 or 40 indexes, would it be better to  
combine them or use a MultiReader? How would we combine them? Thanks  
in advance.

Erik