The S5 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.
The S5 has all-wheel drive to maximize traction under poor conditions, especially in ice and snow. The Z4 doesn’t offer all-wheel drive.
The S5’s standard lane departure warning system alerts a temporarily inattentive driver when the vehicle begins to leave its lane. A lane departure warning system costs extra on the Z4.
The S5 Premium Plus/Prestige has a standard Top View Cameras 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.
Both the S5 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, post-collision automatic braking systems, daytime running lights, rearview cameras, available blind spot warning systems and rear cross-path warning.
For its top level performance in all IIHS frontal, side, rear impact and roof-crush tests, and its standard front crash prevention system, the Insurance Institute for Highway Safety grants the S5 the rating of “Top Safety Pick” for 2017, a rating granted to only 221 vehicles tested by the IIHS. The Z4 has not been tested, yet.