If the TDI has been looked after and serviced properly then it should have plenty of life left in it yet. It really depends on what you are looking for. If you are looking for economy overall then the diesel will be a better choice.
However, bear in mind that diesel is around 10p per litre more expensive and the car is £1000 more so you need to take into consideration how long you will have the car and how long it will take to recover the extra costs. Most people seem to think that you need to do around 12000 miles per year or more to benefit from having a diesel once you have taken into account the (generally) increased cost of buying one and the extra 10p per litre on fuel.
Let us know what you decide to do :)