Aurora is more expensive, more so on the high-end, so it's not entirely clear that it is the best choice for all applications:
db.r3.8xlarge: RDS Aurora $4.640/h, RDS MySQL $3.780/h, RDS PostgreSQL $3.980/h
db.r3.8xlarge: RDS Aurora $4.640/h, RDS MySQL $3.780/h, RDS PostgreSQL $3.980/h
> db.r3.8xlarge: RDS Aurora $4.640/h, RDS MySQL $3.780/h, RDS PostgreSQL $3.980/h
Based on your numbers the difference in pricing between Aurora and Postgres ($4.64 - $3.98) x 24 * 30 = $475/mo. To the company using a db.r3.8xlarge, which has 32-cores and 244 GB of RAM, that's not even a rounding error.
That alone is worth the difference imo.