postgresql versus riak for a global exchange

Hello, I am looking at creating a toy project which may turn into an actual
business if I'm lucky, the ideal is generally just an exchange for people
to trade some type of asset.

I'm looking at using either PostgreSQL or Riak, and I'm wondering if there
are opinions and suggestions that someone would be willing to share with me
when evaluating databases.

This is the first time I've actually considered something besides
PostgreSQL. Riak 2.0 now offers strong consistency and I really respect the
community and the work that has gone into the project. It seems like it
would be easy to replicate across multiple data centers.

Thanks for letting me ask this here!