As an old fart I'll have to agree with the tight arse northerners and consumerist Londoners in that I'd want to make sure I had a job before committing to buying a depreciating asset with an engine that is currently seen as the next worst thing next to North Korean presidents and North American presidents.
With a nod of the head from the right people car trends can suddenly change. A few years ago Diesels were seen as the answer to all the world's problems and were thus rated at zero road tax and the promise of eternal life if you bought one. Current political brainwashing is that Diesels are the equal of war crimes and mass murder and they'll give you lots of money to part exchange them against petrol cars that were the spawn of the devil a few years ago. Petrol cars now only emit fairy dust from their tailpipes and are the current saviour of the world, until they sort out electric cars batteries that will allow you to drive up more than three hills before they need pushing to the nearest wall socket. At that point electric cars will be the saviour of the world and nuclear power stations also run on fairy dust.
There will never be a shortage of lightly used GTD's on dealer forecourts until owning a Diesel grants you automatic status of having to sit in the village stocks for two weeks.
Wait until you've got a decent job and then buy a GTI.
Polo Blue GT's will always fetch a decent part ex price as they're very easy for dealers to sell on again. Well, I say always, in 20 years time they'll maybe not be worth very much depending on whether they've disintegrated or not. But if you've still not got a job in 20 years then that'll be the least of your worries and you'll be glad that you didn't waste £14k on a GTD that depreciated like a stone falling down a waterfall...
Then again, driving around in a GTD while you don't have a job would also be quite nice as it means you won't just be using it to sit in a traffic jam on the way to a job you hate and then back home again while you dream of retiring to somewhere sunny.
Toss a coin for it.