Mongo is specifically designed for horizontal scaling, so it makes it easy to establish replica sets and shards, and it includes an auto-balancer that will distribute your data between the shards. Pg (the database, not the guy!) can do much of this, but it's not quite as built-in.

Which isn't to say there aren't things that Pg is better for. Full text search comes to mind.

Or using the hstore? I would also like to hear an answer on this.

You usually don't have to think about schema. This is the exception, but there are quite a few things you CAN represent with a nested document.

You usually just make an object in whatever format your app wants, save it, and you can write query against it.