Forget the fuel economy of the diesel and look at the performance....
Engine 0-62 50-75 (4th)
318i 10.0 10.9
318d 10.6 9.3
320i 9.0 9.1
320d 8.6 6.7
The diesel will be disappearing over the horozon while the petrol is shifting gears to try and keep up.
But then I am biased towards the diesel
At the end of the day they are all great cars. It's a question of finding the power / toy / style balance that suits you.
But seriously, try and get a drive of them (or come see us at UK2 if there's time) before you make a final decision.