I can't say anything about Diesels vs Petrol, as I've only ever owned Petrol. In terms of various Makes & Models Reliability you should read this Consumer Case Report Article from 2014, which tells about issues with various Makes including Audis
Consumer Reports :-Excessive oil consumption isn't normal
The following is more specific:-Audi Engine Oil Consumption Reading this and doing your own research will tell you to look for either early A3 8P or late A3 8P 1.8TFSI or 2.0 TFSI Petrol, unless there are records to prove that the Revised Pistons & Rings, Timing Chain Tensioner & PCV have already been fitted, it which case, you'll appreciate how nice to drive the A3 8P really are. My only real gripe is that whenever it rains, water floods in, if you have the window down a bit. I bought some aftermarket Heko 4 Piece Weathershields to fix that. I bought a used 2010 A3 8P 1.8TFSI EA888 CDAA Sportwagon with 6 Speed Manual with 203,000 km. It drinks oil (1 litre per 600km) I did ask my Local Australian Audi Dealer if there was anything at all that I should know about any problems this Audi Model before purchasing. The Australian Audi Dealership told me that there were was "nothing known at this time" I even gave them the VIN Number. I bought the car (not from them) in good faith and regret it. It has a Full Audi Service History (FASH) up until 150,000km with the subsequent Services done at Independent Service Places. Did the Audi Dealer know about the aforementioned piston, ring, timing chain tensioner, PCV revisions & inherent problems? I'm going take my car to Audi Dealer for the "Oil Consumption Test" pretty soon and keep my fingers crossed that it FAILS & that Audi will pay for most of the rebuild & repairs. Bottom line is, do your own research, whatever Make or Model you choose. Don't believe everything a Dealer tells you. Get a Warranty with your purchase or at least a fully independent evaluation report done before you buy. Good luck.