I went with the 535i Sport for the following reasons:
- Power and torque -- The N55 engine and 8A transmission are capable of delivering far more performance than you can typically use. There's always plenty of power and it's very easy to find yourself going way too fast. In the northeast with the traffic, stoplights and tightly enforced speed limits, there are only so many opportunities to let it rip during normal driving. When you can enjoy the car's capability, it's a blast.
- The BMW I6 engine -- This is BMW's wheelhouse and what the brand is known for. I have a bias towards products that are part of the company's tradition and have a unique cache. For example, I'd buy a Mont Blanc pen but not a Mont Blanc watch. When I've bought a BMW, I've bought an I6 (I have 3). This is simply my buying pattern.
- Balance -- I prioritized balance between power and handling over straight line speed and acceleration. The 535 is a little lighter and easier to maneuver; the 550 is a bit faster. The balance mattered to me more for everyday driving.
- Trim -- I really liked the sport package wheels type 331 over the 332's. And I like the round exhaust tips better than the trapezoidal. Personal preference.
BMW and the other luxury auto makers continually introduce a higher performing range of models to keep pricing and profits high and stay competitive with each other. Marketing 101. Look how far engines have come over a product generation. The E39 540 was a beast. The F10 535i turbo has more power and is over 0.5 seconds faster 0-60. The F10 550i is in former M5 territory. Both of these cars, as I'm sure you've found, are rockets to drive. Both are quicker than your A6. The 550i is extreme performance and $. The 535i is outstanding performance and excellent bang for the buck.
Hope this helps. Good luck deciding.