Currently, I don't enforce a www.domain.com or domain.com on my site. As such, I have a large number of indexed material under domain.com, but about 1/6th of that is indexed under www.domain.com. When I do site:domain.com, it shows my main page first... if I do site:www.domain.com, it shows some subpage of my site first and no mention of my main page. I am getting ready to do a major implement a major overhaul on my site, lots of mod_rewrite. I already put into place to redirect domain.com to www.domain.com, but wonder if that will be a good thing. Should I error on the side of google index results and redirect everything to domain.com, or should I leave it all alone and don't for force www or non-www for my domain? What is Google's take on this?
Personally, I prefer to see domain.com and I would imagine search engines would as well. It just looks cleaner. The shorter the url, the better.
It makes absolutely no difference to search engines for ranking purposes, but you certainly pick one and redirect the other because you can have problems with google if you don't. I personally use the "www" on all my sites.