The question of which is better, Yahoo or Google, is sort of a trick question.
The better question is: what are you searching FOR?
One of the big problems with the internet is the sheer amount of false information that is available. There are dedicated groups of people with nothing better to do than to associate a particular search result with their particular political, social, or retail point of view. Case in point would be World Of Warcraft. There's a lot of information out there on how to tweak your avatar in the game, complete quests, and so on. There's also a lot of advertiser information focused on selling Gold, selling levels, and so-on.
I perfer to think of the difference as random and non-random search.
Random search is when you only have an idea about what you want to search for, but you don't know where to start, or what the exact terms are.
As a service, Google is hands down better at random search. At the same time, because Google is the better provider, they are also abused the most, and some groups will pay megabucks for search results placements, and others try to game the google search system.
The result is that in practical use Google and Yahoo are actually roughly about the same. Yahoo's random search isn't as good, but their second class status means many of the advertisers and politically minded groups don't bother trying to game their system.
In non-random search, the results are a clear toss-up. If you know what you are looking for, and where you are looking for it, Google and Yahoo are pretty equal at say, searching for the terms fglrx locks up site:phoronix.com. This is also where a lot of stiff competition exists in the search industry, with sites like Technorati limiting their search indexes.
The search question then largely comes down to the companies themselves, and their user-interfaces. Google is open-source friendly, which many of their products under and Open license. Google also finances Mozilla, and sponsors the Summer of Code. As a corporate Champion, Google stands in line with IBM and AMD.
On the other hand, some people think Google is shadier than a Georgia Swamp. Google Chrome, for example, still has no native Linux port, and the licenses attached to some of the programs have driven quite a bit of controversy. Like IBM and AMD, Google cuts some deals that Open-Source supporters aren't really fond of, like AMD's deal with Novell.
Yahoo, as a company though, makes Google look like a saint. Seriously, when was the last time the Unix YIM was updated? and how many times did Yahoo change their chat protocol preventing third party clients like GAIM from connecting?
Seems to me that a lot of people misunderstand how Google's targeted advertising works. The way people talk, you'd think that humans are reading every query and email.
This is absurd, of course; there's way too much happening for a million employees to read even a tiny fraction of it. No, this is why we have machines that perform text analysis and automatically spit back results without any human interaction at all.
Also, to my recollection, you can't actually buy search placement or pagerank on Google. They want you to pay for their advertisements. You end up paying the same amount if you go for the expensive words, but you do so without skewing the results and undermining the veracity of the search. Need proof? Look at a google for "email": GMail ranks eighth. Google News is seventh in a search for "news".