[_Codemonkeyism]

After some years with Mongo I would not use Mongo again (we had some site downs).

It you do not need to push to the client, I'd use Postgres. It just works. For replication and sharding better wait for 10 though.

[cupcakestand]

Can I use Postgres fully schemeless without even defining any collection (table) or db before doing the first write (which is possible with Mongo).

[inimino]

Nope. However, "time to hello world", while certainly a large factor in the success of development tools, is not always the most relevant criteria from an engineering perspective. Cf. PHP.