Was going to post a response to this, but you nailed it.
Anecdotally, we moved off postgres because we could not hire (full time, contact, or consult) anyone who would be considered an expert at postgres replication, and the cost/benefit of switching off compared to growing such an expert favored switching off.
Postgres is my go-to database where a regular backup, rather than warm/live replica, meets availability needs.
Pros:
* Easier to replicate (and hence to scale for some people)
* Easier to find DBA's for
* More popular (twitter, sixapart, etc. use them)
Cons:
* MyISAM is slow as hell
* InnoDB is better but may have licensing issues
* Oracle owns them now, unclear whether it is for better or for worse
PgSQL:
* Better performance of DB engine
* Excellent query planner and analyzer
* Rock-solid in most aspects, with the core team dedicated to putting out a good product
Cons:
* Lack of a canonical, good, production-ready replication method (current choices: Slony, pgPool, Continuent, Londiste)
* Hard to find good DBA's
* Less popular to some extent
There you go. Maybe it boils down to performance (pgsql) vs replication (mysql), but that's just from what I know...