The 550i is no slouch - after all it has approx the same torque as our M5's do. It is very quick off the line - you need to use all of the M5 rev range to be quicker than the 550i. Handling - the 550i feels - like any BMW - that it is on rails, the M5 feels even more stable (if that is the right word) - I would say the M5 suspension and handling is noticeably better.
One note of interest, that I did not expect, the 550i sits lower in the front than the M5 does - an inch or two lower. The 550i does have the M sport package and the sport tranny with paddles. The paddles are different to the M5 pull toward you to up shift, push away to down shift - left and right paddle work exactly the same.
Tires are similar - both 19" wheels - same fronts, 275/30 rear on the 550i, 285/35 on the M5.
550i does have a space saver spare and fog lights. M5's have a nicer headliner (almost "suede like") even though the 550i has the optional anthracite headliner.
If you want an automatic that you don't want to mess with - the 550i tranny is great - much better than the SMG. But, after all we drive ///M cars to drive them - I would not swap the SMG.
The RPi on the 550i sounds great - it was very quiet stock - too quiet. I am looking to put either a Tubi Rumore or Eisenmann Race on the M5. It is not my daily driver so I am not overly concerned about it being too noisy.
Here is a pic of the fronts - the height difference is easier to see in a nose to nose pic - I have one somewhere - will try and dig it out.