Petrol.
The diesel is heavier, so more strain on front suspension components which in turn means more understeer, less well balanced car, the bane of all FWD diesel cars. Front suspension bushes go more readily as do shocks, & front brakes also wear out quicker as well, and are usually more expensive to replace as being uprated to handle the great big lump of iron above them. Tyre wear is also more prominent on diesel cars again due to the extra weight, but also due to the diesels characterises of low down torque, the tyres have got to transmit that to the road and if you want to launch off the line like a petrol car it becomes a constant losing battle for the tyres.
When it comes to service costs it isn't just oil and plugs you have to consider, and as you are talking about a 2nd hand car these things become all the more apparent as they will no doubt be closer to the end of there servicable life.
Drive both, as only you will know which you prefer, you can seek advice but at the end of the day cars are bought with a gut feeling most of the time, not by the head full of other peoples opinions.
Aaron.
