I also had to make a choice.
Over here, a reasonably loaded 550i was more expensive than an S7, and I couldn't really understand why. Yes, interior might look nicer for some but damn the S7 seats... (but not ventilated... :( )

But most importantly... I wanted a sport car. And although there's a lot of litterature about the audi quattro sport differential, I just could not find anything on the 550xi, if their system is at least comparable. I really sought after cornering high speed.

I might have not been super impressed about raw performance and handling the first days (but liked the look though), but since it is EPL stage1 and eurocode chassis parts... I just love it. No regrets.

Now, the new M5 seems to be on demand RWD and AWD. Let's see first detailed reports (and reliability..)