Stick with a petrol, get a GTi and buy a high mile one, lots of exfleet cars around with aprox 80-100K on the clock that will run on for another 100K especially if you avoid the DSG (one less thing to go wrong). You could pick one up for about what yours is worth, mileage kills the value which is why it will work for you.
You also have to ask your self, are you 55 or 25, the A4 and BMW in the spec you are thinking will turn you into a boring old man
. I would only change to the A4 or BMW if they were more powerful e.g. 3ltr diesel (although even quick diesels are a bit boring). So the real question is, Boring and practical or fun and still quiet practical. The BMW 320d is almost GT quick but not GTi quick, that said it does handle great, but too many of them around for my liking, just not special enough.
Also remember, a current diesel owner will advise you to go diesel, listen to the petrol heads! For speed, thrills, fun, handling etc... the GTiiiiiiiiiii still beats the 170, not by a massive amount but enough to make the difference.
Surely you want a GTi, failing that get another VW group car with the GTi engine, I have the Audi A3 Sline with 2ltr TFSI engine, its a peach, should be changing to GTi soon though, love the latests changes to the GTi.
Good luck, hope its a petrol, failing that a Diesel Audi, VW or BMW are still great cars if thats what you want.
