IMO the powertrain on the 535i is the ideal setup for this car. It's the most "BMW" setup of the F10 variations. The I6 is just so unique, sporty and well rounded. It's interesting that you said "sense of occasion" as that's what I really feel from this car and the way it drives. Yes, the motor is outdone by the 550i in every power measure, but the N55's charms are separate from that, it's a certain "feel" you get, a sound, and out of all the cars I drove when I was car shopping, it was the only engine that actually felt individual to a certain brand of car (the other and even far more-so was the 991 Porsche's flat six).
As for the steering I do know the 535i is said to be a superior handling, more fun to drive variation than the 550i, due to the near 50/50 weight distribution and comparable lightness (the 550i is heavier in the front which is not where you want the extra weight if you're choosing).
2015 Porsche Macan S / SportDesign
: White/Black (Alcantara/Leather), PDK, 20" RS Spyder's w/ Colored Crests & Summer Tires, Air Suspension w/ PASM, High-Gloss Black Trim, Premium Package, Comfort Lighting, HD Radio, Blacked Out Sidemarkers, Front Tints.
Recents: '13 BMW 535i M Sport: Black/Black, Premium/Tech, Sport Auto, etc. M-B's: '11/10/06 E350 "Sport". '02 S500 w/ Every Option.