For me, it doesnt matter which has more sites listed. What matters to me is, how much traffic i get from each of the search engines. As of now, around 90% of my traffic comes from google around 4% from yahoo! and 2% from msn rest from others...
Google is certainly king, but I do find MSN likes new sites which at least gives you a trickle of income when you first launch.
Yahoo! has the most pages listed, but lowest quality... I couldn't begin to explain why SE's brag about the number of pages listed, since no one cares about anything past the 3rd SERP (if that) and a lot of cool new sites don't return for querries that they should return for (and maybe will one day). But I haven't been searching lately I'm hooked on del.icio.us.