There's still some advantages to the database you choose. For example ActiveRecord will support the JSON datatype which will come with Postgres 9.2. While they may attempt to make this work in some form for other databases, the performance differences between the two will be quite vast.
The JSON data type is nothing more than a self validating varchar field i.e. fairly pointless.
Better off just validating the JSON yourself in the application layer and maintaining cross database compatibility. To be honest it is rare that you would even need to explicitly validate as when you deserialize into objects it will just fail at that point.
