[Thetawaves]

MySQL - Better performance and tune-ability. Higher connection limits/lower connection costs. Better multi-master and clustered replication story. 2+ decades in extremely high data and availability environments (telecom).

Postgres - Better SQL semantics.

Having supported both in 'bet the business' scenarios, I would choose MySQL hands down. Operating state of the art HA postgres clusters today feels like running MySQL clusters in 2005.