patrick_y[PuristSPro Moderator]
33613
Driven them both...
They're actually extremely similar, the 550i is a tad bit less responsive initially during the start but a very very fast car (445 horsepower). The M5 is even faster, has an even nicer interior, and has the nicer transmission, but otherwise the M5 is very similar to the 550i. The current M5 is a very civilized M5. Very much tuned for comfort. The previous V10 one could beat you up on bad roads and around town (very stiff and head tossing transmission). This one is much more comfortable and much faster.
The previous body style of 5 series; the E60 545i & 550i were very different from the V10 M5. That M5 was difficult to drive around town (mainly transmission), was very stiffly sprung, and was also much noisier when the 545i/550i were very quiet and calm cars. The 545i and 550i (when equipped with Sport package) had active roll suspension, which smoothes out the ride by preventing body lean in curves (this is now known as "ARS" on the current 5-series and is a far less common $3500 option) that the M5 did without. The active roll suspension is a 7-series technology trickle down; it works by having a motor twist the anti-roll bars in the suspension to prevent the car from leaning in the curve. It's a fully active technology, not a passive technology, but it doesn't take into account everything; for instance if the banking of the curve is unusual for the curve, the computer probably doesn't take that into account. Someday, there will be a stereo camera that will be mounted to communicate to the car's suspension computer. The system designed to increase the handling limit of the car, but at the same time really reduces the feel (and the satisfaction) from the driving experience. Porsche has the same technology now called PDCC (Porsche Dynamic Chassis Control) and some purists and magazine editors have stated they don't prefer it even though it does contribute to significantly faster lap times. So, overall the difference between the 545i/550i and M5 was quite substantial in 2004-2010 model years, but in the current 5-series iteration, the gap is much less substantial.