I respectfully disagree...
Sorry, but I respectfully disagree that the high-end Audi products built by Quattro GmbH, the more exclusive division of Audi (sort of like AMG to Mercedes-Benz) have better build quality and better reliability...the statistics simply don't bear that out.
If anything, the higher-end Audi RS models are more susceptible to maintenance issues AND are more costly to maintain because of 1) their greater complexity and 2) the perceived higher status that these more exclusive cars carry.
Having lived the German car experience, the fact of the matter is that most of those marques have built their reputations on innovation, engineering, design, speed, handling prowess...yes...all of the above...but NOT on reliability.
Anyone who takes the time to research average reliability records over time will see that Audi, VW, and Mercedes are not particularly good (not even the most expensive models), BMW is only slightly better, and the one exception that actually comes out pretty good -- but still not as unassailable in terms of reliability as a Japanese car -- is Porsche. Porsches actually do pretty well.
Now, to be clear...the question of reliability is a very personal decision. I will be the first person to agree that most of the vehicles from all of the brands above are lovely, desirable, (mostly) impressive, and exciting to drive. But one would be foolish to think that they lead the pack in quality. It's the other things one is paying for...the excitement, the engineering innovation, and the soul. But those come at a price.
Many of the folks who are rich enough to afford these cars are willing to pay that price, because they can literally afford to throw their money away on maintenance at a dealership that offers complimentary cafe lattes while you wait to be parted from your money.
But I assure you, unless you are one of those people, i.e. if you are an average bloke like most of us here (and like me), and you aspire to the excitement of an Audi, BMW or Mercedes as a kind of "stretch" goal, you'd better know what you are getting into. It is not easy to afford, and unless you are truly a do-it-yourself, roll-up-your sleeves kind of mechanic, you will pay dearly to maintain those cars.
Again, I loved driving my Audi and it was beautiful. However, at a certain point, as an average person who also has a mortgage, a job, kids to raise, and who also wants some funds to enjoy great hobbies like mountain biking, after a while a vehicular money pit starts to feel really stupid. I felt like a chump for sinking so many dollars, month after month, into....a car. That's right, that's all it is, at the end of a day. A car.
I still remain an avid auto enthusiast, and I am much more selective about what I'd include on a list of cars to seriously consider buying.
On the other specific questions...
It's true that a BMW 330i Touring is not in the same class as an RS4; an M3 wagon, if such a beast existed, would be a better comparison. However, that's not to say that a 330i isn't a very respectable car. And the 3-series BMWs do have decent reliability. Really, let's be serious...unless you're racing someone else, do you think you're really going to feel the difference between 4.5 seconds vs. 5.5 seconds from 0-60? No, you won't. Both will put a silly smile on your face, just the same.
Me, I'd still pick a Porsche or a Japanese performance car over any of the others, any day of the week.