Hi,
Check my signature - I am an Audi fan.
But, when it came to choosing a diesel compact executive car, I went for BMW, after months of in-depth research.
The reasons are simple:
1. BMW has a lot more real world experience (good and bad) with turbo diesels in an SA context. They burned their fingers badly with the the E46 320d turbo failures in 2000-2001 (look for an old Car Magazine article called "Pop goes the turbo" by Jake Venter) and their product development team ensured that each iteration of the 320d built upon the mistakes of the older model. The 320d is arguably the best balanced performance/consumption/reliability German in the SA market (MB c220d and Audi 2.0tdi are close 2nd and 3rd respectively).
2. The A4 is FWD and the torque steer is off putting in a premium car. BMW (and MB) handle their torque much better/refined, being RWD.
3. Audi SA is a dick to its customers when it comes to known engine issues out of Freeway Plan ("motorplan"). While the rest of the world have claimed out of plan costs from Audi AG for rectifying the oil consumption fault, Audi SA will make you pay for an oil consumption test and then in 4/5 cases reject your claim.
4. The Multitronic gearbox is crap. Full stop.
5. The Audi manual box is good (I had a B8 1.8T with this), but automatic is the way to go for any diesel car, unless you like constant gear changing to stay in the that tight sweet spot rev range (unlike a decent petrol motor). In this case, an Auto BMW (no crap CVT or DSG system) is the way to go.
6. BMW interiors are crap compared to Audi (my 2010 B8 feels more premium than my 2017 F30!), but the BMW has a logical simplicity and solidity to everything that should make it more reliable in the long run.
I would say go for the E90 320d, with some good aftermarket warranty. Or, if you are going Audi, then go for the 1.8TFSI manual (B8.5) - you should have some plan left.