From a performance view I'd say the 320 still has the edge, but I see where you're coming from; potentially there's more scope for tuning the 230k.
The V6 has about 30bhp more and gets to 60mph about a second quicker; there's about 35Nm more torque.
Going by the W209, they're not a great performance car. - I think the ride (even with "AMG sport" suspension) is a little too wallowy and as a result I don't have a huge amount of confidence throwing it in to the corners. The ESP seems to have two modes: "I don't think you meant to do that so I'm cutting the power" (when it's on); and "you're on your own pal, I hope you like going sideways" when it's off.
Don't think that I don't like mine. It's a great GT cruiser and surges along the motorway in a pleasing manner; it just doesn't like twisty B roads. - I think I also need to get used to having lots of low-end torque and not high-end power (like a turbocharged engine), so I'm probably stomping the throttle more than I need to. When I squeeze it gently, it takes off from the lights.
Maybe someone with more experience of the W208 has something to offer?