The decision what database to connect if driven by costs and needed functionality. I assume at the low end H2 is no discussion. But this database has no capabilities of failover and redundant configuration. Therefore not suitable for running an enterprise application.
If you want to add these enterprise capabilities you can choose ORACLE and MSSQL for sure, but this will add royalties to these companies for licenses increasing the cost of ownership. PostgreSQL is a good alternative delivering these enterprise feature with no additional costs. Anyhow, also PostgreSQL will have limitations regarding scale like other relational databases. In this case you need to switch over to one of the IoT databases like CASSANDRA. Again here PTC offers a very cost effective and scale able solution out of the box.
I assume in future we sill see additional storage provider interfaces delivered by partners and SI. E.g. SAP HANA, ORACLE, MS-SQL I see in the front row of interest.
First thing, PostgreSQL is an open source db with advanced features that comes at no cost. It is very productive and easy-to-use database. For more details, please refer the article at Why Thingworx-PostgreSQL.
When compared with ThingWorx Neo db, PostgreSQL provides accessibility to the inner datatables and schemes. As Neo database is a "Graph" database and getting into the data schemes is difficult as it involves cipher. Neo db has its own pros and cons, but it has less scope for scalability. As Thingworx is more committed for Scalability/High Availability, so ThingWorx is towards using PostgreSQL.