I'm curious enough to propose an experiment. Three sites are created with an identical topic and similar domain names, so that the domain name isn't a factor.
One is XF, one if VB and the third is IPB. No change to themes/skins/templates. That all remains "as installed". No add-ons either, except for free community SEO add-ons. Seed content for each sire in unique, but of the same number and quality.
Leverage the built-in and community released SEO tools (nothing that charges a fee, must all be free) and then "release them into the wild" for six months, providing only normal administrative care and feeding.
At the end of six months compare the traffic, content quantity and quality, page rankings, key word positions, etc for all three and declare a winner. Is anyone else curious about what the results might be?