Thanks a lot for the detailed reply, this is why I love HN!
1-2 years back I saw a benchmark stating that PG works better than MySQL on I/O constrained AWS instances, so I didn't expect this ("I've learned something today").
I think Postgres is a better default choice because you are likely to be safer and for it to behave properly with your data. In certain cases, apparently Mysql might be faster for certain workloads, and with certain tradeoffs. That's the kind of informed decision you can make once you're scaling to that size.
1-2 years back I saw a benchmark stating that PG works better than MySQL on I/O constrained AWS instances, so I didn't expect this ("I've learned something today").