I am one of those Ex-petrol Converts. I remember when my company were going to force us all to drive diesel company cars a couple of years ago - people were actually going to resign if they did [:O]
Having said that the new generations of diesel are soooooo much better. I did find that the 2.0Tdi I have was a litlle noisier than I had expected, and is certainly louder than my wifes Mondeo(cough,cough) 2.o Tdci GhiaX, but then that may have something to do with sound deadening materials?
Anyway, you should drive both and then make up your own mind (i am sure you would anyway), but don't fall into the trap of thinking the diesel is more expensive to buy than a petrol. Yes, you will pay more for it initially, but as well as the MGP savings, which are only really relevant to high mileage driver, you will also realise a much better residual value in the current 2nd hand market. This may change though if they carry on stinging diesel drivers at the pumps at the rate at which they are. I saw diesel for 10pence more than petrol yesterday - was'nt so long ago that it was more than 10 pence cheaper.
Lastly, on yoour test drive don'nt make the mistake of revving the nuts out of the diesel, use it's torque and gears to gain the best speed.
Good luck with your choice.