I have both. The 335d feels lighter than the M3 because the torque kicks in with the engine at low RPM and not trying very hard.
However, getting back to the OP, the M sport package on the d is nothing like the M3. The M3 is much firmer and I easily go over speed bumps that cause the d to bounce severely and hit the bump stops. The real M3 suspension is far better. The steering wheel, floor mats, and door sills on the 335d M sport package do nothing to overcome this.
I love getting 40 mpg in the d, but there is no substitute for the rumble of a real M3 V8 through an Eisenmann race. Car enthusiasts hear the M3 and nod their heads in envious approval. That's what the wannabe is missing out on.