its only what i heard
something to do with the cost of running a TDI to a GTi (standard models) you'd get more for your money with petrols in shorter distances unless your doing massive commutes? where then the TDi would come into its own, its never made much sense to me as if you get good MPG on a long trip then surely shorter trips would be better? 
This only applies to cars where the premium cost of buying a diesel needs to be offset against a petrol. If you are buying a car that has had that initial cost offset already (ie an old car that costs essentially the same to buy as a petrol), then it's irrelevent.
So the bonus of buying an old diesel, it will always be cheaper to run than a petrol per mile driven.
The downside is twofold. You have to drive a diesel, and it costs sh!t loads when it breaks.