I think this whole conversation is making a mountain out of a molehill. When you're building sites which have enormous performance requirements, you take what you have and you force it to scale. You don't need to compare one tool to the other and find the perfect benchmark. Whatever you use, I promise you, you can make it perform as fast and scale as far as you need it to.
So just pick the tool that fits your requirements the best and stick with it. That being said, I also urge you to pick something free and open-source if you can. Some of the ways big sites have leveraged MySQL to the extent they have is due to the custom patchsets they apply. Closed-source solutions may not provide similar hotfixes or customization (and when it comes it's months later than they claimed they'd have it ready)
So just pick the tool that fits your requirements the best and stick with it. That being said, I also urge you to pick something free and open-source if you can. Some of the ways big sites have leveraged MySQL to the extent they have is due to the custom patchsets they apply. Closed-source solutions may not provide similar hotfixes or customization (and when it comes it's months later than they claimed they'd have it ready)