I don't know why I don't like PostgreSQL.
There's nothing particular about it. It doesn't do anything offensive. It doesn't smell. It doesn't mix stripes and dots. It doesn't eat the last of the ice cream.
But having tried pgsql a dozen times in a dozen projects I have always ripped it out in favor of a MySQL fork. Plenty of huge shops use pgsql and it's fine. I can't say don't use it. All I can say with pgsql is your mileage may vary and it if works for you, that's fantastic. There's just something about it that I just don't like and I cannot recommend working with it.
The MySQL Community started fleeing the main branch the moment Oracle bought MySQL and made it their own. Oracle and the Open Source community don't mix. It's bad juju there. Oracle wants heavily paying corporate partners who are locked into an Oracle/Solaris/Java-based stack and the Open Source Community wants to push forward with wacky new database engines which just happily grow and breed and bud and turn into new and wonderful things. The result is a real parting of the ways from Oracle and everyone else.
Percona's fork comes with Percona's tools and Percona's support. It has the new XtraDB engine to replace InnoDB and a bunch of consulting. I've heard of great success with Percona's fork of MySQL and even greater success with unleashing Percona's ninjas on a non-functional web application. It seems very not bad. I admit, I have not used Percona's fork of MySQL in any real environment but I know of those who have and it's a reasonable choice.
I've personally had much better success with MariaDB -- the fork the original MySQL team now develops. It's less of a consulting vehicle than a technology vehicle as it has all sorts of interesting new storage engines and tools and tweaks and a focus on improving performance and reliability while pushing the engine more into the Big Data space. It's definitely stable and works well in production. I like it, I have found it Good for some measurable amount of Good and can say with confidence that one cannot go wrong with using MariaDB as their main database back end.