I think it depends what you value.
+More road feel, better traction and higher cornering speeds.
+Prestige, ability to impress people
- Everything is optional and to get the Porsche vaguely close in equipment to an 80k 550Xi you would spend 120k
- Lot of technology Porsche doesn't offer (surround cameras, HUD, HD Nav screen, Apps, etc.)
- Flip side of better traction and cornering is that the car is too in control and therefore lacks edge when driving. BMW is more scary and fun.
+ Faster in straight line with the $3K Dinan upgrade - will pull out over a Panamera noticeably
+ Much better technology
+ More equipment and luxuries for the price
+ More fun - feels a little bit edgy relative to the Porsche from a driver standpoint. It's not as composed as the Porsche.
- Looks the same as a humble $45k 528i. Very low key.
- Less road feel and slower cornering (still not bad)
2014 Porsche 911 Turbo S
2015 Mercedes C300 4MATIC