Alexa wrote about this in a blog last year (Oct.30th, 2007). Here's a link... http://awis.blogspot.com/2007/10/google-vs-youtube.html (Scroll down to the 4th paragraph) Basically google uses multiple domain names where as yahoo uses subdomains. Where you put all of google's international domains together google is still #1.
Probably until about 6 months ago, Yahoo's results were much much better than Google. People more heavily target google for blackhat seo, so you end up with the results going to crap as soon as someone figures the algo out. Then when they change the algo, it gets better. Yahoo remained consistent for years, giving me pretty good results for all my searches. Only recently has it started to decline dramatically.