The answer to that would certainly be relevant to advertising strategies, and budgets. I'll have to research my own logs now that I've asked...
i don't think there is a definitive answer to your question, just test, review and test again till you hit the sweet spot.
briggidere
With Google's new 3 ad format, I would expect their PPC CTR to be a bit higher now.
I would also expect Yahoo's to be a bit higher based on a few small tweaks they've made (do a search at Yahoo on a 800x600 monitor).