The six-cylinder engine is far more refined and powerful; but, the N57 is prone to bottom-end damage, which may be rather expensive to repair. As the engine ages, its prevalence is increasing.
The four-cylinder engine will be the B47, which has demonstrated greater reliability than its N47 predecessor.
I recommend test-driving the 420d with the ZF8 automatic gearbox to assess its performance, as it is not a sluggish vehicle.