I think it depends on your area of the US where I live most everything is Equifax. I'm glad I would croak if it was the evil experian.
No, there all scum Seriously though, creditors randomly pull 1 or 2 out of the 3, some pull all 3! It is nearly impossible to predict 100% who will pull what, but goto PsychDOCS http://groups.yahoo.com/group/whopulledmycredit to see some patterns
Here in Dallas, it seems as though almost everyone pulls Experian...which just happens to be my worst score now. My two best cards, Chase and BofA, pulled TU, as well as my credit union I refinanced my truck through. Guess I'll be asking companies who they pull before I apply with them for now on.