Over the last few years I have seen one sure thing. If you do well with Google, you do equally well with Yahoo - fairly obvious as Yahoo takes its main results from Google. Thus a search for "XYZ" which comes 23rd on Google will come 23rd on Yahoo, the difference being the sponsored sites at the top and base of the Yahoo pages.
That still holds true for my sites that have been around a while, however the newer sites seem to progress further and faster with Yahoo!
In one particular case "drinking Water", my site
http://www.drinking-water.co.uk is 2nd on a Yahoo.co.uk UK search while I have dropped completely from Google.co.uk (UK only) for that search term!
Adding a word "services" that identifies my site, I get to be No1 on both Google.co.uk (UK only) and .com so it is unlikley to be a case that I have been dropped by Google for any assumed dodgy
SEO on my part. This isn't the only instance I have noticed for a newer web site.