Cost
The BMW M6 is tremendously costlier than the BMW 3 Series with respect to MSRP.
Utility
The M6's larger trunk will accommodate more large things than the BMW 3 Series.
Neither the BMW 3 Series nor the BMW M6 has a marked advantage regarding seating capacity.
Comfort
While the front cabin in the BMW M6 offers a bit more head room than the 3 Series, there really isn't much of a difference.
The 3 Series offers more head- and leg-room to your passengers than the BMW M6 does.
Dimensions
The BMW M6 is somewhat heavier than the BMW 3 Series.
Your wallet will hurt more every time you fill up the BMW M6 than the BMW 3 Series because of its bigger tank.
The BMW 3 Series and the BMW M6 compete for the same parking spaces.
Performance
With that kind of horsepower, the BMW M6 is a true speedster.
As far as horsepower, the M6 packs quite a wallop and will knock the 3 Series down a few rungs. On the flip side, note that high horsepower engines often command higher insurance premiums.
Handling
The M6 has substantially wider tires than the BMW 3 Series. Remember that bigger tires could improve handling, while narrower tires can improve gas mileage.
With their comparable turning circles, the BMW 3 Series and the BMW M6 most definitely handle approximately the same into and out of tight spots.
Drivetrain
The BMW M6 has a more massive engine than the BMW 3 Series. Keep in mind that smaller engines typically get better gas mileage, but a smaller engine usually has to work a little harder.
Torque is roughly equivalent to acceleration, and in this respect, the BMW M6 is tremendously more powerful than the 3 Series.
Because the BMW M6's maximum torque is reached at high RPMs, you'll have to shift gears a lot, especially when passing.
Overview
The BMW M6 and the BMW 3 Series have the same basic warranty.