Which Calculate Field expression_type should be used?

I discovered today that VB syntax is not working in the x64 processor when I run Calculate Field. I get this error

Executing: CalculateField lyr XXVAL 1 VB #
Failed to execute. Parameters are not valid.
ERROR 000989: Tool CalculateField cannot use VB expressions for service

So, it looks like if I want my tools to run in x64 arcpy, I must avoid the default VB parser.

Does anyone know which is better to use: the "PYTHON" or "PYTHON_9.3" parser? I'm wondering if there is any performance or compatibility benefit for using "PYTHON" if you don't need to access shape properties. I would think PYTHON_9.3 would be more efficient as it returns objects instead of string representations.

I'm hoping someone on the geoprocessing team can give us advice so we can write CalculateValue expressions in a way that will be the most efficient and forward-compatible...