I would choose MySQL because I'm more familiar with it and it's good enough for what I need. PostgreSQL might be able to give me better benchmarks but it won't have any meaningful benefit for me the developer or for the user.
Benchmarks are not the reason people choose PostgreSQL over MySQL. I have never even bothered to benchmark the two for any project that could have used either.
Features, better ACID (at least historically) and maybe better standards compliance.
The big one for me is being able to run schema changes in transactions, which makes it easy to roll back a failed migration.
