The 2.6 M103 six is the most common pick for 'sweet spot,' described as smooth, strong and a 'different league' from the four. But 2.3 M102 owners push back, calling the four light and zippy and adequate, while a minority dismiss it as an 'anemic 4 banger.' Notably some argue the 2.6 is not far off the vaunted 16-valve because it makes more torque.
The 2.6 inline-six vs the 2.3 four-cylinder
Mercedes-Benz 190E / 190D (W201) · driving