How easy it is to install and maintain. How performant it is. How good are the tools associated with it.

I've yet to run into a situation where MySQL was holding me back. Since the only thing postgres has got going for it, from my standpoint, is that my friends prefer it and it is cooler… I think that until I have a better reason I will stick to the one with the better toolset.

Right now, Sequel Pro is that tool. I use it almost every single day.