Bach 3C vs Marcinkiewicz 6S
The Marcinkiewicz 6S has a wider cup diameter, the Marcinkiewicz 6S has a larger throat opening.
Cup Diameter
The Marcinkiewicz 6S has a cup diameter of 16.84 mm / 0.6630 in compared to 16.30 mm / 0.6417 in on the Bach 3C — a difference of 0.54 mm / 0.0213 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Marcinkiewicz 6S has a wider throat (3.73 mm / 0.1469 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.