This was exactly our my case. I did 10 servers going flat out calling into one (beefy at the time) MySQL. I was amazed at how far you could go with that. Then we had replication at the time built in, and so did read replicas. For free and somewhat hacky software the scaling was solid.

When I started playing with postgresql I had no clue why folks liked it. My memory may be bad, but we used UPSERT and replication. On Postgresql at least through 8 you could not upsert or stream replication.