Tops lowered to worship the summer sun, we stomp on gas pedals and shoot down the rural roads outside of Ann Arbor. The SLK and Z4, despite weighing some 3400- and 3500-pounds respectively, reward our indulgence with effortless acceleration and exhaust notes raspy enough to dispel any remaining notions that these are mere poseurs. We're impressed by the smoothness and flexibility of the SLK's big V-6, which responds enthusiastically regardless of the tachometer needle's position. That said, this is still BMW's dominion. Its inline-six reacts instantly to commands of the throttle and at full howl drowns out the Mercedes.
Both cars might be described as having seven-speed automatics, but the BMW unit has the key modifier, "dual-clutch." As a result, the SLK simply can't match the ferociousness of the Z4's upshifts nor the immediacy of its downshifts, especially when we apply multiple taps to the steering-wheel-mounted paddle. We suspect this quick shifting, as much as the 33-hp advantage, explains why the Z4 is about a half second faster to 60 mph, according to factory estimates. Still, we're more annoyed with the Mercedes' software than the hardware. Regardless of whether it's in economy or sport mode, the gearbox proves too eager to upshift and isn't nearly proactive enough in downshifting under hard braking. There is a manual mode, but even that won't keep it from upshifting if you happen to hit the rev limiter - not what we want during cornering.
The Z4's gearbox does not entirely escape criticism. We still think BMW's insistence that each paddle should be able to call up either upshifts or downshifts (tap forward for upshift, pull back for upshift) is confusing. And for all its sporting credentials, the Z4, like the SLK, often refuses to hold a gear at redline, even with the transmission in manual mode and all the adjustable electronic settings dialed up to eleven.
Heavy steering is not always good steering
The SLK and Z4 are both fantastic handlers in the modern idiom, which is to say they rely on high levels of grip, excellent body control, and some high-tech wizardry. The sDrive35is has adaptive dampers with the usual sport and comfort settings (the SLK350 can be equipped similarly, but only as a special order option that was not on our test vehicle). In the real world, at least, it's tough to discern a substantial difference in body control, especially since the Z4 is so stiffly sprung to begin with.