The AMG SL has standard head airbag curtains for front and rear seats which act as a forgiving barrier between the driver and outboard passenger's upper bodies and the window and pillars. Combined with high-strength steel door beams and lower side airbags this system increases head protection in broadside collisions. The Z4 doesn't offer side airbag protection for the head.
Over 200 people are killed each year when backed over by motor vehicles. The AMG SL has a standard Maneuvering Brake that uses rear sensors to monitor for objects to the rear and automatically applies the brakes to prevent a collision. The Z4 doesn’t offer backup collision prevention brakes.
The AMG SL has all-wheel drive to maximize traction under poor conditions, especially in ice and snow. The Z4 doesn’t offer all-wheel drive.
The AMG SL has a standard Surround View Camera to allow the driver to see objects all around the vehicle on a screen. The Z4 only offers a rear monitor and front and rear parking sensors that beep or flash a light. That doesn’t help with obstacles to the sides.
The AMG SL has a standard blind spot warning system that uses sensors to alert the driver to objects in the vehicle’s blind spots where the side view mirrors don’t reveal them. A system to reveal vehicles in the Z4’s blind spot costs extra.
To help make backing out of a parking space safer, the AMG SL has standard Rear Cross-Traffic Alert and Rear Cross Traffic Brake automatically engages the brakes to help avoid a collision. BMW charges extra for Cross Traffic Warning on the Z4 and the Z4’s Cross Traffic Warning does not include automatic braking.
Both the AMG SL and the Z4 have standard driver and passenger frontal airbags, front side-impact airbags, driver and front passenger knee airbags, front seatbelt pretensioners, four-wheel antilock brakes, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, daytime running lights, rearview cameras, driver alert monitors and available lane departure warning systems.
The Mercedes AMG SL weighs 762 to 991 pounds more than the BMW Z4. The NHTSA advises that heavier cars are much safer in collisions than their significantly lighter counterparts.