mileage would be my deciding factor, even though the diesel will do much better MPG if you only doing 5-7k a year you won't have any real saving when compared with a petrol. Service costs are usually more for a diesel (parts more expensive), parts in general are more expensive but are usually more reliable.
To compare I have had to replace a fuel pump in 2 of my astras, one being diesel one being petrol. The petrol fuel pump cost me £50 brand new the diesel pump cost over £700 for a refurb!!!
The purchase cost is usually more for a diesel too so again if your doing low mileage the saving now will be the petrol due to the price of diesel compared with petrol.
I'm guessing as you are comparing the two condition is pretty much identical, as already mentioned check the MOT history online for both, service history and how they drive.