MySQL historically had more support for HA/clustering than Postgres. Recently, there's been a lot of progress on integrating Postgres clustering into the core,

MySQL historically had more support for HA/clustering than Postgres. Recently, there's been a lot of progress on integrating Postgres clustering into the core, to the point where it's mature, but perhaps not as battle-tested. Not a reason to choose MySQL for a startup, I think, but if you've got a cluster working on MySQL and a clear understanding of its pitfalls, there's no real reason to switch to Postgres.