According to this Reuters article, a Yahoo-Google combination would face more antitrust scrutiny than a Yahoo-Microsoft merger:
Google held a 59.2 percent share of the U.S. Web search market in February, compared with Yahoo’s 21.6 percent and Microsoft’s 9.6 percent, according to research firm comScore.
Those numbers would give a Yahoo-Google combination an 80.8 percent market share.
Combine Yahoo and Microsoft and their market share is only 31.2 percent, which is less than Google’s numbers.
