What AWS instance to choose for write-heavy PostgreSQL database server
Asking "What AWS instance to choose for write-heavy PostgreSQL database server" is somewhat like asking "which spoon should I use to cut up this loaf of bread".

The answer is: don't use a spoon, use a knife. Or in this case, real hardware like a dedicated server with a batch of good quality SSDs and a decent raid controller that has a battery-backed write-back cache. Or at least a VPS with very strong I/O performance guarantees. 

If in doubt, benchmark.

That said, you can make AWS perform significantly better than the defaults by using provisioned I/O EBS volumes on EBS-optimized instances, improving performance while retaining data durability. Alternately, you can sacrifice durability in exchange for more performance by using a high I/O instance with the database on the instance store; in this case you will need a careful backup and replication scheme because all your data will be lost if your instance stops or crashes.

If you need more assistance, contact me directly.