If I were starting from zero knowledge, I'd choose Postgres, but you're not. Unless you require on particular database extensions (full-text search, PostGIS, etc), or you're operating at the kind of scale where all bets are off, it won't matter very much; use the one your team has more expertise with. MySQL sucked twenty years ago, but it's had twenty years of people beating on it, so it largely sucks in precise, well-understood ways now. This is nearly as good as being good.