Thanks to all of you for your advice. As far as I can tell, both cars are materially in very decent condition and are similarly specced apart from the engine. As I said, the prices are identical.
In fact, the stats say that the petrol 1.6FSI is (very slightly) quicker 0-60mph than the diesel 1.9TDI. However, I've heard good things about VW diesel engines - even the weaker ones.
I suppose my question is: do you think that the extra control from the diesel's increased torque is worth a 30000 mile penalty?
I'm not exactly a boy racer - just a normal mix of urban, rural and motorway driving, but I live in a pretty hilly area and am fed up of driving a car where it takes 10 seconds after putting pressure on the pedal for any sort of response!