For enhanced safety, the front and rear seat shoulder belts of the Mercedes AMG CLA have pretensioners to tighten the seatbelts and eliminate dangerous slack in the event of a collision and force limiters to limit the pressure the belts will exert on the passengers. The Audi S3 doesn’t offer pretensioners for its rear seat belts.
The AMG CLA offers an optional Surround View System to allow the driver to see objects all around the vehicle on a screen. The S3 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 CLA 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 S3’s blind spot costs extra.
To help make backing out of a parking space safer, the AMG CLA has standard Rear Cross-Traffic Alert and Active Brake Assist automatically engages the brakes to help avoid a collision. Audi charges extra for Rear Cross-Traffic Assist on the S3.
The AMG CLA’s driver alert monitor detects an inattentive driver then sounds a warning and suggests a break. According to the NHTSA, drivers who fall asleep cause about 100,000 crashes and 1500 deaths a year. The S3 doesn’t offer a driver alert monitor.
Both the AMG CLA and the S3 have standard driver and passenger frontal airbags, front side-impact airbags, driver and front passenger knee airbags, side-impact head airbags, four-wheel antilock brakes, all wheel drive, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, daytime running lights, rearview cameras and available lane departure warning systems.

