It's generally best to ground the neutral as close to the source as possible, as this provides the most efficient path to ground. This will also keep voltage drop between the grounding point and X0 terminal as low as possible, which will keep the 3 phases better centered around ground. In practice, it's only going to make a few volts worth of difference, especially if you're only talking about 6', but it's still more efficient to bond at the transformer.
I would recommend bonding at the transfomer.
